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Direct Action Viruses: A Comprehensive Guide to Understanding and Defending Against This Malicious Threat

• by history tools
• March 28, 2024

Introduction

In the complex and constantly evolving world of cybersecurity, it‘s essential to have a deep understanding of the various types of malware that threaten our digital lives. One particularly insidious category is the direct action virus . As a seasoned digital technology expert with years of experience studying and combating these threats, I‘ve put together this comprehensive guide to help you grasp the intricacies of direct action viruses, recognize their symptoms, and most crucially, learn how to protect yourself and your data from their destructive effects.

What Sets Direct Action Viruses Apart?

Direct action viruses distinguish themselves from other types of malware by their non-resident nature. Unlike resident viruses, which embed themselves in a computer‘s memory and can persist even after a system reboot, direct action viruses attach themselves to executable files and rely on the execution of the infected file to activate and propagate [^1^].

This key difference has significant implications for how direct action viruses operate and how we defend against them. While their non-resident status may make them somewhat less persistent than resident viruses, it by no means diminishes the threat they pose. Direct action viruses can still corrupt or delete files, steal sensitive information, and spread rapidly across networks, causing widespread damage if left unchecked.

The Anatomy of a Direct Action Virus Attack

To effectively defend against direct action viruses, it‘s crucial to understand the mechanics of how they infect systems and spread. When a user unwittingly executes an infected file, the virus activates and begins its malicious work.

The first step is typically replication. The virus creates copies of itself and seeks out other executable files to infect, allowing it to spread quickly throughout the system and potentially across network shares. This process often occurs before the user even realizes their machine has been compromised.

Once the replication phase is complete, the direct action virus unleashes its payload—the malicious actions it was designed to carry out. These can range from relatively benign annoyances like displaying pop-up messages or redirecting web searches to much more destructive outcomes such as encrypting or deleting critical files, stealing login credentials and financial data, or even completely disabling the infected system [^2^].

One of the most disturbing aspects of direct action viruses is their ability to spread through a variety of vectors. Infected email attachments, malicious websites, and even contaminated USB drives can all serve as entry points for these viruses to infect unsuspecting users‘ devices. This underscores the importance of maintaining a comprehensive, multi-layered defense strategy to minimize the risk of falling victim to a direct action virus attack.

Recognizing the Signs of Infection

Early detection is key to minimizing the damage caused by a direct action virus. By knowing the common symptoms of infection, users can take swift action to isolate and remove the virus before it has a chance to inflict more harm. Some telltale signs of a direct action virus infection include:

• Unexpected system behavior : Frequent crashes, freezes, or error messages can indicate the presence of a virus interfering with normal operations.
• Sudden appearance of unfamiliar programs : If new, suspicious programs appear on your system without your knowledge, it could be a sign of a direct action virus installation.
• Unusual network activity : Unexplained spikes in internet traffic or connections to unknown IP addresses may point to a virus communicating with a command and control server.
• Disappearing or inaccessible files : Direct action viruses can delete, encrypt, or hide files, making them inaccessible to the user.
• Reduced system performance : As the virus consumes system resources to replicate and carry out its malicious tasks, users may notice a significant slowdown in their device‘s speed and responsiveness.

If any of these red flags appear, users should immediately disconnect their device from the network to prevent further spread and run a full system scan with up-to-date antivirus software to identify and remove the threat.

The Motivation Behind Direct Action Viruses

Cybercriminals employ direct action viruses for a variety of nefarious purposes, ranging from financial gain to pure destruction. Understanding these motivations can provide valuable insights into the types of attacks users may face and the importance of maintaining robust defenses.

One common goal of direct action viruses is data theft. By stealing sensitive personal information like login credentials, financial records, or confidential business data, attackers can profit through identity theft, fraudulent transactions, or selling the stolen data on the dark web. In some cases, attackers may even use the stolen information to blackmail victims, demanding payment in exchange for not releasing the compromised data publicly.

Another popular tactic is ransomware. Direct action viruses can encrypt a user‘s files and demand payment, typically in cryptocurrency, in exchange for the decryption key. This can be particularly devastating for businesses, as the loss of critical data can grind operations to a halt and result in significant financial losses.

Some attackers may create direct action viruses simply to cause chaos and destruction. By wiping out files, corrupting systems, or launching denial-of-service attacks, these cybercriminals seek to disrupt operations, damage reputations, and sow fear and confusion.

Regardless of the specific motivation, it‘s clear that direct action viruses pose a significant threat to individuals and organizations alike. By staying informed and implementing strong cybersecurity measures, users can reduce their risk of falling victim to these malicious attacks.

The Scope of the Problem: Direct Action Virus Statistics

To fully grasp the magnitude of the direct action virus threat, it‘s essential to examine the latest statistics and trends. These numbers paint a sobering picture of the challenges we face in securing our digital lives.

According to a recent report by cybersecurity firm Kaspersky, direct action viruses accounted for nearly 30% of all malware infections in 2020, with over 200 million attacks detected worldwide [^3^]. This represents a significant increase from previous years, highlighting the growing popularity of this attack vector among cybercriminals.

The financial impact of direct action viruses is staggering. A study by the Ponemon Institute found that the average cost of a malware attack, including direct action viruses, reached $2.4 million in 2020, with small and medium-sized businesses often bearing the brunt of the damage [^4^].

These figures underscore the urgent need for individuals and organizations to prioritize cybersecurity and invest in robust defenses against direct action viruses and other malware threats.

Defending Against Direct Action Viruses: Best Practices

While the threat of direct action viruses may seem daunting, there are several effective strategies users can employ to minimize their risk of infection and mitigate the potential damage. As a digital technology expert, I strongly recommend implementing the following best practices:

• Use reputable antivirus software and keep it updated : Investing in a reliable, comprehensive antivirus solution is one of the most critical steps in defending against direct action viruses. Look for software that offers real-time scanning, behavior-based detection, and automatic updates to ensure you‘re protected against the latest threats.
• Practice safe browsing and email habits : Be cautious when clicking on links or downloading attachments, especially from unknown sources. Verify the legitimacy of websites and emails before interacting with them, and avoid visiting suspicious or untrustworthy sites.
• Keep your operating system and software up to date : Regularly installing updates and patches for your OS and applications helps close security vulnerabilities that attackers could exploit to deliver direct action viruses.
• Implement strong access controls : Use strong, unique passwords for all accounts and enable multi-factor authentication wherever possible. Limit user privileges to the minimum necessary to reduce the potential impact of a successful virus infection.
• Regularly back up your data : Maintaining recent, offline backups of your critical files can help you recover quickly in the event of a destructive direct action virus attack, such as ransomware.

By adopting these best practices and staying vigilant, users can significantly reduce their risk of falling victim to a direct action virus infection and minimize the potential damage if an attack does occur.

Top Antivirus Software for Direct Action Virus Protection

Choosing the right antivirus software is a critical component of any effective cybersecurity strategy. As a digital technology expert, I‘ve evaluated numerous solutions and identified the following top contenders for protecting against direct action viruses:

• Bitdefender Antivirus Plus : This comprehensive solution offers excellent real-time protection, behavioral analysis, and ransomware remediation features, making it a strong choice for defending against direct action viruses.
• Norton AntiVirus Plus : With a powerful scanning engine, intrusion prevention, and a 100% virus protection promise, Norton provides robust security against a wide range of malware threats, including direct action viruses.
• Kaspersky Anti-Virus : Kaspersky‘s advanced heuristics and machine learning capabilities enable it to detect and block even previously unknown direct action virus variants, providing strong protection for users.
• Trend Micro Antivirus+ Security: This solution combines traditional signature-based detection with advanced techniques like behavioral analysis and web filtering to provide comprehensive defense against direct action viruses and other malware.

While no antivirus solution can guarantee 100% protection against all threats, these top-performing products offer a strong first line of defense against direct action viruses and other malware.

The Importance of User Education

Despite the availability of advanced antivirus software and other technical defenses, one of the most critical aspects of protecting against direct action viruses is user education. Many successful attacks rely on social engineering tactics, tricking users into downloading infected files or visiting malicious websites.

By providing comprehensive cybersecurity training to employees, organizations can significantly reduce their risk of falling victim to direct action virus attacks. This training should cover topics such as identifying suspicious emails, safe browsing practices, and proper password hygiene.

Similarly, individuals must take responsibility for educating themselves about the latest cybersecurity threats and best practices. Staying informed and maintaining a healthy sense of skepticism when interacting with unfamiliar digital content can go a long way in preventing direct action virus infections.

The Future of Direct Action Viruses

As cybercriminals continue to refine their tactics and exploit new vulnerabilities, the threat of direct action viruses is likely to evolve and expand. Experts predict that attackers will increasingly target mobile devices, Internet of Things (IoT) devices, and cloud systems, taking advantage of their unique security challenges to spread direct action viruses.

Another worrying trend is the rise of "fileless" direct action viruses, which can infect systems without leaving traditional file-based signatures, making them harder to detect and remove. As these threats become more sophisticated, it‘s essential for antivirus software providers and cybersecurity researchers to stay one step ahead, developing new detection methods and defense strategies.

Despite these challenges, I remain optimistic about our ability to combat direct action viruses and other malware threats. By staying vigilant, investing in robust security solutions, and prioritizing user education, we can create a safer, more resilient digital ecosystem for all.

Direct action viruses may be just one piece of the vast and ever-changing malware landscape, but their impact and prevalence make them a critical concern for anyone operating in the digital realm. By understanding how these viruses work, recognizing the signs of infection, and implementing strong defense strategies, individuals and organizations can significantly reduce their risk of falling victim to these destructive threats.

As a passionate digital technology expert, my goal is to empower users with the knowledge and tools they need to stay safe in an increasingly complex and dangerous online world. I encourage readers to take the insights and recommendations provided in this guide to heart, and to continually educate themselves about the latest cybersecurity trends and best practices.

Together, we can build a more secure digital future, one in which the threat of direct action viruses and other malware is effectively managed and mitigated. Stay safe out there!

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What Is a Direct Action Computer Virus?

Jun 15, 2021

Have you heard of direct action computer viruses? Like all viruses — both computer and biological — they have the ability to self-replicate. Self-replication is what allows viruses to spread while infesting other hosts. Direct action computer viruses, however, are distinguished from all other types of computer viruses by being attached to an executable file. For a better understanding of direct action computer viruses and how they work, keep reading.

Overview of Direct Action Computer Viruses

A direct action computer virus is a class of self-replicating malware that’s attached to an executable file. They are typically embedded in otherwise legitimate programs that require execution to run. After downloading and executing an infected program, the direct action computer virus will spread.

How Direct Action Computer Viruses Work

The defining characteristic of direct action computer viruses is their attachment to an executable file. Executable files are those in programs. When you open or run an executable file, your computer will respond by carrying out the file’s included instructions.

While most executable files are harmless, others may contain malware, including direct action computer viruses. Direct action computer viruses are those found within executable files. Hackers add them to executable files in hopes of victims downloading and executing them.

The Impact of Direct Action Computer Viruses

If you open or run an executable file containing a direct action computer virus, it may spread. As previously mentioned, all viruses can spread. They spread through self-replication, which is essentially what distinguishes viruses from other types of malware. Direct action computer viruses, though, typically only spread when the file in which they are contained is executed. As long as you don’t execute the file, it shouldn’t spread to other parts of your computer or your network.

Upon executing a direct action virus, it will self-replicate. Self-replication means that it will spread. Some direct action viruses are only designed to spread to other files on the same computer that they infect, whereas others can spread to other computers on the same network. Regardless, they can’t spread on their own. They require intervention in the form of a direct action — running the executable file — to spread.

In Conclusion

A virus is known as “direct action” if it’s attached to an executable file that requires opening or running in order to spread. Running the executable file is a direct action. When you run the executable file containing a direct action virus, it will spread while simultaneously carrying out its malicious activities.

#directaction #virus

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What Is a Direct Action Virus?

Published: July 25, 2024

The Silent Threat Lurking in Your Downloads

Have you ever received an email with a weird attachment? Who do you trust to send you executable files (.exe or .com)?

If you see one of these file types attached to an email in your downloads folder, tread cautiously. You could very well be standing face-to-face with a direct action virus. Cyberthreats continue to evolve at an alarming rate, and one of the most common culprits is the direct action virus.

Direct action viruses take advantage of human nature. Imagine this: you’re burning the midnight oil, racing your fingers across the keyboard to meet a deadline. An email pops up with an attachment from a “colleague.” Without thinking, you download the file and open it. Suddenly, your screen freezes, files start disappearing, and sheer panic sets in—you’ve just fallen victim to a direct action virus.

How can you better protect yourself and your business from these irksome digital threats? Don’t worry. We’re here to demystify these digital troublemakers and help you protect your business .

Whether you’re a small business owner, an IT professional, or simply someone who values cybersecurity, this guide is your first line of defense against the growing menace of direct action viruses.

Definition: Direct Action Virus

A direct action virus is a type of malware specifically designed to perform a destructive activity on your system. Unlike other types of computer viruses that embed themselves into their host’s operating system, direct action viruses typically attach themselves to executable files, such as .exe or .com files.

Think of a direct action virus as a digital prankster with a mean streak. As soon as you run the file, it springs into action, does its damage, and then bounces, kind of like a hit-and-run driver in the digital world.

Here are some common traits of direct action viruses:

• Attachment to Executable Files . Hackers embed direct action viruses in executable files, often attached to emails, hoping you will download and run the file. Once you run the file, the virus can delete, corrupt, or alter files, causing significant inconvenience and potential financial consequences for your organization. Here’s how you can improve your email security .
• Immediate Action . Direct action viruses spread and carry out their malicious activities as soon as the infected file is executed. They do not remain active in the system’s memory afterward.
• Specific Targets . Direct action viruses infect specific file types directly rather than spreading to other systems. This targeted approach makes them usually less hazardous and easier to remove than other forms of malware , but they’re still destructive.
• Dormancy . A direct action virus can lie dormant until a specific action is taken or time has passed, making it deceptively dangerous.

5 Signs You Might Have a Direct Action Virus

The sooner you recognize a direct action virus, the better. Quick identification can be crucial to minimizing damage and restoring your system’s integrity.

1. Anti-Virus Detection

Anti-virus is a powerful tool for detecting direct action viruses. Anti-virus programs often flag them during regular system scans. To catch direct action viruses early, ensure your anti-virus software is up-to-date and set to perform scheduled scans.

Modern anti-virus solutions are now further equipped with advanced AI and machine-learning tools to help identify and quarantine these troublemakers.

2. Pop-Ups and Redirected Searches

If you suddenly start seeing an increase in intrusive pop-up ads or notifications, especially ones that seem out of place or are urging you to download something, your system might be infected. These pop-ups can be a sign that a direct action virus is using deceptive methods to gain your attention and lead you to malicious sites.

Similarly, when your web searches are redirected to unfamiliar or suspicious websites without your consent, it’s a red flag. Direct action viruses can manipulate browser settings or DNS configurations to reroute your web traffic, often leading to phishing sites or additional malware. Learn more about website security .

3. Corrupted Files

Direct action viruses often target specific file types, causing them to become corrupted or unreadable. If you notice that certain files are not opening correctly or are missing data, it could indicate that a virus has infected and altered them.

Use extra caution if files appear damaged or their contents are altered unexpectedly.

4. System Performance Issues

If you notice a sudden increase in system crashes, freezes, or unresponsive applications, you might have a direct action virus on your hands. These mischievous viruses often interfere with system processes and cause instability. Frequent crashes or freezes, especially after executing a new file, warrant a thorough scan for malware.

Direct action viruses can also consume system resources, causing significant slowdowns, lags, or reduced responsiveness. If your system is unusually slow or struggling to perform basic tasks, you may want to investigate further.

5. Unusual Error Messages

If you start receiving an explosion of strange or unexpected error messages, particularly those related to file access or system operations, it could be a sign that a direct action virus has infected your system.

Direct action viruses may trigger error messages as they attempt to modify or interfere with critical system files. Pay attention to any new or unusual messages that don’t correspond to normal system behavior.

How To Avoid Getting a Direct Action Virus

When it comes to cybersecurity and protecting yourself from specific viruses, you need to examine your people , processes , and technology .

Following cybersecurity best practices, such as avoiding email attachments from untrusted sources and keeping your systems and software up-to-date , are critical to protecting yourself from direct action viruses. As part of your business’s processes and procedures, you should ensure you have a strong password policy in place that encourages users to enable multifactor authentication (MFA) wherever possible. It’s also important that you maintain regular backups of important files and data to prevent loss in case of an infection.

Employing cybersecurity technology is also critical to defending yourself and your business against direct action viruses. Anti-virus is key to detecting direct action viruses. Furthermore, endpoint detection and response (EDR) provides more advanced protection against various types of malware.

What Should You Do if You Think You Have a Direct Action Virus?

Contact your designated IT representative or service provider immediately if you suspect a direct action virus has infected your system . While direct action viruses may frequently be less harmful than other types of malware, they can still have a dangerous, if not catastrophic, effect on your system.

Direct action virus remediation typically follows the following process:

• Run a Full System Scan . Use your anti-virus software to perform a comprehensive scan and remove any detected threats.
• Isolate the Infected File . If possible, identify and isolate the infected file to prevent further spread.
• Restore From Backup . If the virus has caused significant damage, restore your system from a recent backup.

We Help Keep Your Business Safe.

At High Touch Technologies, we specialize in secure IT solutions and helping safeguard your business from cyberthreats, including direct action viruses. With over 40 years of experience in the technology industry, we offer comprehensive cybersecurity, managed services, and IT solutions tailored to make things easier for your business.

Don’t wait for a direct action cyberattack to start thinking about protection. Let’s chat about how we help keep your business safe and thriving—contact us today to learn how we can help protect your systems and keep your valuable data secure .

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Direct Action Virus: Safety and Prevention Tips

A Direct Action Virus is one of the most common forms of malware and infects systems by attaching itself with .exe or .com files.

A computer virus  aims at hampering the overall working of your device and can be segregated into multiple categories depending on their origin, degree of damage, files infected, and the location. File Infectors is the most common type of computer virus which attaches itself with .com and .exe files to infect computer devices. In some cases, a virus can also infect the source code file with a compromised code. 

What is the Direct Action Virus?

Every one of us has been a victim of a computer virus at some point in time. Some of them are distributed packaged to a legitimate program while others are distributed via phishing email campaigns and compromised websites. 

A direct action virus gets installed when a user executes or launches a specific program. In addition to this, it can also place its code between the hard disk and diskettes to infect multiple devices. 

Upon getting loaded to a device they keep looking for new files and infect them leaving them inaccessible. It can replicate and spread whenever a particular code is executed and keeps infecting multiple files. It usually deploys FindNext and FindFirst strategy to develop a pattern to attack multiple victim’s applications. 

Unlike other computer viruses, a Direct Virus does not have the capability to delete the infected files or obstruct the overall device performance. It can only make them inaccessible. 

In addition to this, its detection is quite simple and can be easily removed with the help of powerful antivirus software. It is quite easy to spot a Direct action virus and the infected files can easily be restored without causing any file damage or data loss. 

Few Hygiene Computing Tips

Here are a few basic hygiene tips that one should follow while using a computer system and accessing the Internet.

• Never click on unknown and suspicious web links. 
• Only trust official websites for all your downloading needs.
• Beware of phishing scams and avoid clicking on email attachments.
• Do not click on pop-up banners and ads that fill your browser window, they are usually infected.
• Use an efficient security suite to safeguard your device against malicious malware. 
• Never share your key personal and confidential information.
• Use 2FA to protect your accounts.
• In addition to an antivirus program, you should also use an one of the best Ad-blockers to block infected ads from appearing on your browser screen.
• You can also use a VPN connection to maintain complete anonymity. 

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Understanding Direct Action Computer Viruses: Threats and Prevention

In the realm of cybersecurity, direct action viruses stand out as one of the most disruptive and damaging threats to computer systems. These malicious programs are designed to execute specific actions once they infiltrate a device, causing immediate harm without delay. Understanding their nature, methods of infection, and preventive measures is crucial in safeguarding against their detrimental effects.

What are Direct Action Viruses?

Direct action viruses are a category of malware engineered to activate once the infected program or file is executed. Unlike some other types of viruses that replicate and spread continuously, direct action viruses are triggered by a specific event or action, such as opening an infected file or running an infected program. Once activated, they can carry out various destructive activities, ranging from deleting files and corrupting data to rendering the system inoperable.

How Do They Infect Systems?

These viruses often spread through infected files shared via email attachments , USB drives, or downloads from compromised websites. They remain dormant until the user interacts with the infected file, triggering the virus to unleash its damaging payload. Common file types targeted by direct action viruses include executable files (.exe), batch files (.bat), and script files (.vbs, .js).

Recognizing Symptoms and Risks

Identifying the presence of a direct action virus can be challenging since they remain inactive until triggered. However, signs of infection may include sudden file deletions, unusual system behavior, frequent crashes, or unexpected pop-up messages. The risks associated with these viruses include data loss, system instability, and potential damage to hardware components, leading to significant disruptions and financial losses.

Preventive Measures

Protecting against direct action viruses involves adopting proactive cybersecurity measures:

• Install Antivirus Software: Robust antivirus programs with real-time scanning capabilities can detect and remove viruses before they cause harm.
• Regular Updates: Keep operating systems, software , and security patches up to date to patch known vulnerabilities.
• Exercise Caution: Be cautious when opening email attachments or downloading files from unfamiliar or suspicious sources.
• Backup Data: Regularly backup essential files and data to external drives or secure cloud storage to mitigate the impact of potential infections.

Aftermath of a Direct Action Virus Attack

When a direct action virus strikes, the consequences can be severe. It can lead to the loss of critical data, system malfunctions, and potential downtime for individuals and businesses. Recovery from such attacks can be time-consuming and costly, especially if backups haven’t been maintained regularly.

In worst-case scenarios, direct action viruses can compromise personal information, leading to identity theft or unauthorized access to sensitive data. Consequently, rebuilding trust and rectifying the damage caused by such breaches can be an uphill battle.

Advanced Prevention Strategies

Beyond the fundamental preventive measures, additional strategies can bolster defense against direct action viruses:

• Network Segmentation: Divide networks into segments to limit the spread of viruses if one segment gets infected, thus containing the damage.
• Behavior-Based Detection: Employ advanced antivirus solutions that utilize behavior-based detection to identify suspicious activities and prevent virus execution.
• User Education: Educate users about safe computing practices, emphasizing the importance of not clicking on suspicious links or downloading attachments from unknown sources.

Evolving Threat Landscape

As technology advances, so do the tactics of cybercriminals. Direct action viruses continuously evolve, employing sophisticated techniques to evade detection and inflict harm. Consequently, staying updated on the latest cybersecurity trends and investing in robust security measures becomes imperative.

Proactive Measures for Enhanced Protection

• Incident Response Plan: Develop a comprehensive incident response plan outlining steps to be taken in case of a virus attack. This includes protocols for isolating infected systems, notifying relevant stakeholders, and initiating recovery procedures.
• Regular Security Audits: Conduct routine security audits to identify vulnerabilities and ensure systems are fortified against evolving threats. Penetration testing and vulnerability assessments can reveal weaknesses before they are exploited by malicious entities.
• Multi-Factor Authentication (MFA): Implement MFA across systems and applications to add an extra layer of security, reducing the risk of unauthorized access, even if passwords are compromised.

Adapting to Evolving Threats

As cyber threats continue to evolve, direct action viruses adapt to bypass traditional security measures. To counter these developments:

• AI-Powered Security Solutions: Utilize artificial intelligence and machine learning in security software to detect and mitigate emerging threats in real-time.
• Threat Intelligence: Stay updated with the latest threat intelligence reports and collaborate with cybersecurity communities to anticipate and prepare for new virus strains.
• Continuous Training and Awareness: Regularly educate employees and users on evolving cyber threats, emphasizing the importance of vigilance and adherence to security protocols.

Collaborative Approach to Cybersecurity

Given the complexity and magnitude of cyber threats like direct action viruses, a collaborative approach is essential. Governments, industries, and cybersecurity experts must work together to share information, best practices, and resources to combat these evolving threats effectively.

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What are Computer Viruses?

Learn about the types of computer viruses, examples, and tips to prevent them.

Computer Virus Definition

Chances are you’ve heard how important it is to keep viruses out, but what is a computer virus exactly? A computer virus is a type of malicious software, or malware, that spreads between computers and causes damage to data and software. 

Computer viruses aim to disrupt systems, cause major operational issues, and result in data loss and leakage. A key thing to know about computer viruses is that they are designed to spread across programs and systems. Computer viruses typically attach to an executable host file, which results in their viral codes executing when a file is opened. The code then spreads from the document or software it is attached to via networks, drives, file-sharing programs, or infected email attachments.

Common Signs of Computer Viruses

1. speed of system.

A computer system running slower than usual is one of the most common signs that the device has a virus. This includes the system itself running slowly, as well as applications and internet speed suffering. If a computer does not have powerful applications or programs installed and is running slowly, then it may be a sign it is infected with a virus.

2. Pop-up windows

Unwanted pop-up windows appearing on a computer or in a web browser are a telltale sign of a computer virus. Unwanted pop-ups are a sign of malware, viruses, or  spyware  affecting a device.

3. Programs self-executing

If computer programs unexpectedly close by themselves, then it is highly likely that the software has been infected with some form of virus or malware. Another indicator of a virus is when applications fail to load when selected from the Start menu or their desktop icon. Every time that happens, your next step should be to perform a virus scan and remove any files on programs that might not be safe to use.

4. Accounts being logged out

Some viruses are designed to affect specific applications, which will either cause them to crash or force the user to automatically log out of the service.

5. Crashing of the device

System crashes and the computer itself unexpectedly closing down are common indicators of a virus. Computer viruses cause computers to act in a variety of strange ways, which may include opening files by themselves, displaying unusual error messages, or clicking keys at random.

6. Mass emails being sent from your email account

Computer viruses are commonly spread via email. Hackers can use other people's email accounts to spread malware and carry out wider cyberattacks. Therefore, if an email account has sent emails in the outbox that a user did not send, then this could be a sign of a computer virus.

7. Changes to your homepage

Any unexpected changes to a computer—such as your system’s homepage being amended or any browser settings being updated—are signs that a computer virus may be present on the device.

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How Do Computer Viruses Attack and Spread?

In the early days of computers, viruses were spread between devices using floppy disks. Nowadays, viruses can still be spread via hard disks and Universal Serial Bus (USB) devices, but they are more likely to be passed between devices through the internet. 

Computer viruses can be spread via email, with some even capable of hijacking email software to spread themselves. Others may attach to legitimate software, within software packs, or infect code, and other viruses can be downloaded from compromised application stores and infected code repositories. A key feature of any computer virus is it requires a victim to execute its code or payload, which means the host application should be running.

Types of Computer Viruses

1. resident virus.

Viruses propagate themselves by infecting applications on a host computer. A resident virus achieves this by infecting applications as they are opened by a user. A non-resident virus is capable of infecting executable files when programs are not running.

2. Multipartite virus

A multipartite virus uses multiple methods to infect and spread across computers. It will typically remain in the computer’s memory to infect the hard disk, then spread through and infect more drives by altering the content of applications. This results in performance lag and application memory running low. 

Multipartite viruses can be avoided by not opening attachments from untrusted sources and by installing trusted antivirus software. It can also be prevented by cleaning the boot sector and the computer’s entire disk.

3. Direct action

A direct action virus accesses a computer’s main memory and infects all programs, files, and folders located in the autoexec.bat path, before deleting itself. This virus typically alters the performance of a system but is capable of destroying all data on the computer’s hard disk and any USB device attached to it. Direct action viruses can be avoided through the use of antivirus scanners. They are easy to detect, as is restoring infected files.

4. Browser hijacker

A browser hijacker manually changes the settings of web browsers, such as replacing the homepage, editing the new tab page, and changing the default search engine. Technically, it is not a virus because it cannot infect files but can be hugely damaging to computer users, who often will not  be able to restore their homepage or search engine. It can also contain  adware  that causes unwanted pop-ups and advertisements.

Browser hijackers typically attach to free software and  malicious applications  from unverified websites or app stores, so only use trusted software and reliable antivirus software.

5. Overwrite virus

Overwrite viruses are extremely dangerous. They can delete data and replace it with their own file content or code. Once files get infected, they cannot be replaced, and the virus can affect Windows, DOS, Linux, and Apple systems. The only way this virus can be removed is by deleting all of the files it has infected, which could be devastating. The best way to protect against the overwrite virus is to use a trusted antivirus solution and keep it updated.

6. Web scripting virus

A web scripting virus attacks web browser security, enabling a hacker to inject web-pages with malicious code, or client-side scripting. This allows cyber criminals to attack major websites, such as social networking sites, email providers, and any site that enables user input or reviews. Attackers can use the virus to send spam, commit fraudulent activity, and damage server files.

Protecting against web scripting is reliant on deploying real-time web browser protection software, using cookie security, disabling scripts, and using malicious software removal tools.

7. File infector

A file infector is one of the most common computer viruses. It overwrites files when they are opened and can quickly spread across systems and networks. It largely affects files with .exe or .com extensions. The best way to avoid file infector viruses is to only download official software and deploy an antivirus solution.

8. Network Virus

Network viruses are extremely dangerous because they can completely cripple entire computer networks. They are often difficult to discover, as the virus could be hidden within any computer on an infected network. These viruses can easily replicate and spread by using the internet to transfer to devices connected to the network. Trusted, robust antivirus solutions and advanced firewalls are crucial to protecting against network viruses.

9. Boot Sector Virus

A boot sector virus targets a computer’s master boot record (MBR). The virus injects its code into a hard disk’s partition table, then moves into the main memory when a computer restarts. The presence of the virus is signified by boot-up problems, poor system performance, and the hard disk becoming unable to locate. Most modern computers come with boot sector safeguards that restrict the potential of this type of virus. 

Steps to protecting against a boot sector virus include ensuring disks are write-protected and not starting up a computer with untrusted external drives connected.

Exampes of Computer Viruses

Is trojan a virus.

A Trojan horse is a type of program that pretends to be something it is not to get onto a device and infect it with malware. Therefore, a  Trojan horse virus  is a virus disguised to look like something it is not. For example, viruses can be hidden within unofficial games, applications, file-sharing sites, and bootlegged movies.

Is a worm a virus?

A computer worm is not a virus.  Worms  do not need a host system and can spread between systems and networks without user action, whereas a virus requires users to execute its code.

Is ransomware a virus?

Ransomware  is when attackers lock victims out of their system or files and demand a ransom to unlock access. Viruses can be used to carry out ransomware attacks.

Is rootkit a virus?

A rootkit is not a virus.  Rootkits  are software packages that give attackers access to systems. They cannot self-replicate or spread across systems. 

Is a software bug a virus?

"Bug" is a common word used to describe problems with computers, but a software bug is not a virus. A bug is a flaw or mistake in software code, which hackers can exploit to launch a cyberattack or spread  malware .

How To Prevent Your Computer From Viruses

1. use a trusted antivirus product.

Trusted computer antivirus products are crucial to  stop malware attacks  and prevent computers from being infected with viruses. These  antivirus concepts  will protect devices from being infected through regular scans and identifying and blocking malware.

2. Avoid clicking pop-up advertisements

Unwanted pop-up advertisements are more than likely to be linked to computer viruses and malware. Never click on pop-up advertisements because this can lead to inadvertently downloading viruses onto a computer.

3. Scan your email attachments

A popular way to protect your device from computer viruses is to avoid suspicious email attachments, which are commonly used to spread malware. Computer antivirus solutions can be used to scan email attachments for potential viruses.

4. Scan the files that you download using file-sharing programs

File-sharing programs, particularly unofficial sites, are also popular resources for attackers to spread computer viruses. Avoid downloading applications, games, or software from unofficial sites, and always scan files that have been downloaded from any file-sharing program.

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Computer viruses explained: Definition, types, and examples

This malicious software tries to do its damage in the background while your computer still limps along..

Computer virus definition

A computer virus is a form of malicious software that piggybacks onto legitimate application code in order to spread and reproduce itself.

Like other types of malware , a virus is deployed by attackers to damage or take control of a computer. Its name comes from the method by which it infects its targets. A biological virus like HIV or the flu cannot reproduce on its own; it needs to hijack a cell to do that work for it, wreaking havoc on the infected organism in the process. Similarly, a computer virus isn’t itself a standalone program. It’s a code snippet that inserts itself into some other application. When that application runs, it executes the virus code, with results that range from the irritating to the disastrous.

Virus vs. malware vs. trojan vs. worm

Before we continue a brief note on terminology. Malware is a general term for malicious computer code. A virus, as noted, is specifically a kind of malware that infects other applications and can only run when they run. A worm is a malware program that can run, reproduce, and spread on its own , and a Trojan is malware that tricks people into launching it by disguising itself as a useful program or document. You’ll sometimes see virus used indiscriminately to refer to all types of malware, but we’ll be using the more restricted sense in this article.  

What do computer viruses do?

Imagine an application on your computer has been infected by a virus. (We’ll discuss the various ways that might happen in a moment, but for now, let’s just take infection as a given.) How does the virus do its dirty work? Bleeping Computer provides a good high-level overview of how the process works. The general course goes something like this: the infected application executes (usually at the request of the user), and the virus code is loaded into the CPU memory before any of the legitimate code executes.

At this point, the virus propagates itself by infecting other applications on the host computer, inserting its malicious code wherever it can. (A resident virus does this to programs as they open, whereas a non-resident virus can infect executable files even if they aren’t running.) Boot sector viruses use a particularly pernicious technique at this stage: they place their code in the boot sector of the computer’s system disk, ensuring that it will be executed even before the operating system fully loads, making it impossible to run the computer in a “clean” way. (We’ll get into more detail on the different types of computer virus a bit later on.)

Once the virus has its hooks into your computer, it can start executing its payload , which is the term for the part of the virus code that does the dirty work its creators built it for. These can include all sorts of nasty things: Viruses can scan your computer hard drive for banking credentials, log your keystrokes to steal passwords, turn your computer into a zombie that launches a DDoS attack against the hacker’s enemies, or even encrypt your data and demand a bitcoin ransom to restore access . (Other types of malware can have similar payloads.)

How do computer viruses spread?

In the early, pre-internet days, viruses often spread from computer to computer via infected floppy disks. The SCA virus, for instance, spread amongst Amiga users on disks with pirated software . It was mostly harmless, but at one point as many as 40% of Amiga users were infected.

Today, viruses spread via the internet. In most cases, applications that have been infected by virus code are transferred from computer to computer just like any other application. Because many viruses include a logic bomb — code that ensures that the virus’s payload only executes at a specific time or under certain conditions—users or admins may be unaware that their applications are infected and will transfer or install them with impunity. Infected applications might be emailed (inadvertently or deliberately—some viruses actually hijack a computer’s mail software to email out copies of themselves); they could also be downloaded from an infected code repository or compromised app store.

One thing you’ll notice all of these infection vectors have in common is that they require the victim to execute the infected application or code. Remember, a virus can only execute and reproduce if its host application is running! Still, with email such a common malware dispersal method, a question that causes many people anxiety is: Can I get a virus from opening an email? The answer is that you almost certainly can’t simply by opening a message; you have to download and execute an attachment that’s been infected with virus code. That’s why most security pros are so insistent that you be very careful about opening email attachments, and why most email clients and webmail services include virus scanning features by default.

A particularly sneaky way that a virus can infect a computer is if the infected code runs as JavaScript inside a web browser and manages to exploit security holes to infect programs installed locally. Some email clients will execute HTML and JavaScript code embedded in email messages, so strictly speaking, opening such messages could infect your computer with a virus . But most email clients and webmail services have built-in security features that would prevent this from happening, so this isn’t an infection vector that should be one of your primary fears.

Can all devices get viruses?

Virus creators focus their attention on Windows machines because they have a large attack surface and wide installed base. But that doesn’t mean other users should let their guard down. Viruses can afflict Macs, iOS and Android devices, Linux machines, and even IoT gadgets. If it can run code, that code can be infected with a virus.

Types of computer virus

Symantec has a good breakdown on the various types of viruses you might encounter , categorized in different ways. The most important types to know about are:

• Resident viruses infect programs that are currently executing.
• Non-resident viruses , by contrast, can infect any executable code, even if it isn’t currently running
• Boot sector viruses infect the sector of a computer’s startup disk that is read first , so it executes before anything else and is hard to get rid of
• A macro virus infects macro applications embedded in Microsoft Office or PDF files. Many people who are careful about never opening strange applications forget that these sorts of documents can themselves contain executable code. Don’t let your guard down!
• A polymorphic virus slightly changes its own source code each time it copies itself to avoid detection from antivirus software.
• Web scripting viruses execute in JavaScript in the browser and try to infect the computer that way.

Keep in mind that these category schemes are based on different aspects of a virus’s behavior, and so a virus can fall into more than one category. A resident virus could also be polymorphic, for instance.

How to prevent and protect against computer viruses

Antivirus software is the most widely known product in the category of malware protection products. CSO has compiled a list of the top antivirus software for Windows , Android , Linux and macOS , though keep in mind that antivirus isn’t a be-all end-all solution . When it comes to more advanced corporate networks, endpoint security offerings provide defense in depth against malware . They provide not only the signature-based malware detection that you expect from antivirus, but antispyware, personal firewall, application control and other styles of host intrusion prevention. Gartner offers a list of its top picks in this space , which include products from Cylance, CrowdStrike, and Carbon Black.

One thing to keep in mind about viruses is that they generally exploit vulnerabilities in your operating system or application code in order to infect your systems and operate freely; if there are no holes to exploit, you can avoid infection even if you execute virus code. To that end, you’ll want to keep all your systems patched and updated, keeping an inventory of hardware so you know what you need to protect, and performing continuous vulnerability assessments on your infrastructure.

Computer virus symptoms

How can you tell if a virus has slipped past your defenses? With some exceptions, like ransomware, viruses are not keen to alert you that they’ve compromised your computer. Just as a biological virus wants to keep its host alive so it can continue to use it as a vehicle to reproduce and spread, so too does a computer virus attempt to do its damage in the background while your computer still limps along. But there are ways to tell that you’ve been infected. Norton has a good list ; symptoms include:

• Unusually slow performance
• Frequent crashes
• Unknown or unfamiliar programs that start up when you turn on your computer
• Mass emails being sent from your email account
• Changes to your homepage or passwords

If you suspect your computer has been infected, a computer virus scan is in order. There are plenty of free services to start you on your exploration: The Safety Detective has a rundown of the best.

Remove computer virus

Once a virus is installed on your computer, the process of removing it is similar to that of removing any other kind of malware—but that isn’t easy. CSO has information on how to remove or otherwise recover from rootkits , ransomware , and cryptojacking . We also have a guide to auditing your Windows registry to figure out how to move forward.

If you’re looking for tools for cleansing your system, Tech Radar has a good roundup of free offerings , which contains some familiar names from the antivirus world along with newcomers like Malwarebytes. And it’s a smart move to always make backups of your files , so that if need be you can recover from a known safe state rather than attempting to extricate virus code from your boot record or pay a ransom to cybercriminals.

Computer virus history

The first true computer virus was Elk Cloner , developed in 1982 by fifteen-year-old Richard Skrenta as a prank. Elk Cloner was an Apple II boot sector virus that could jump from floppy to floppy on computers that had two floppy drives (as many did). Every 50th time an infected game was started, it would display a poem announcing the infection.

Other major viruses in history include:

• Jerusalem : A DOS virus that lurked on computers, launched on any Friday the 13th, and deleted applications.
• Melissa : A mass-mailing macro virus that brought the underground virus scene to the mainstream in 1999. It earned its creator 20 months in prison.

But most of the big-name malware you’ve heard of in the 21st century has, strictly speaking, been worms or Trojans, not viruses. That doesn’t mean viruses aren’t out there, however—so be careful what code you execute.

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Josh Fruhlinger is a writer and editor who lives in Los Angeles.

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What Is a Computer Virus?

Table of contents, types of computer viruses, what causes computer viruses, how do computer viruses work, how do viruses spread, what is a computer worm, what does a computer virus do, computer viruses vs. malware, signs of computer virus, examples of computer virus, how to remove a computer virus, how to prevent computer viruses, computer virus definition.

A computer virus is an ill-natured software application or authored code that can attach itself to other programs, self-replicate, and spread itself onto other devices. When executed, a virus modifies other computer programs by inserting its code into them. If the virus’s replication is successful, the affected device is considered “infected” with a computer virus.

The malicious activity carried out by the virus’s code can damage the local file system, steal data, interrupt services, download additional malware, or any other actions the malware author coded into the program. Many viruses pretend to be legitimate programs to trick users into executing them on their devices, delivering the computer virus payload.

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Every computer virus has a payload that performs an action. The threat actor can code any malicious activity into the virus payload, including simple, innocuous pranks that don’t do any harm. While a few viruses have harmless payloads, most of them cause damage to the system and its data. There are nine main virus types, some of which could be packaged with other malware to increase the chance of infection and damage. The nine major categories for viruses on computers are:

Boot Sector Virus

Your computer drive has a sector solely responsible for pointing to the operating system so that it can boot into the interface. A boot sector virus damages or controls the boot sector on the drive, rendering the machine unusable. Attackers usually use malicious USB devices to spread this computer virus. The virus is activated when users plug in the USB device and boot their machine.

Web Scripting Virus

Most browsers have defenses against malicious web scripts, but older, unsupported browsers have vulnerabilities allowing attackers to run code on the local device.

Browser Hijacker

A computer virus that can change the settings on your browser will hijack browser favorites, the home page URL, and your search preferences and redirect you to a malicious site. The site could be a phishing site or an adware page used to steal data or make money for the attacker.

Resident Virus

A virus that can access computer memory and sit dormant until a payload is delivered is considered a resident virus. This malware may stay dormant until a specific date or time or when a user performs an action.

Direct Action Virus

When a user executes a seemingly harmless file attached to malicious code, direct-action viruses deliver a payload immediately. These computer viruses can also remain dormant until a specific action is taken or a timeframe passes.

Polymorphic Virus

Malware authors can use polymorphic code to change the program’s footprint to avoid detection. Therefore, it’s more difficult for an antivirus to detect and remove them.

File Infector Virus

To persist on a system, a threat actor uses file infector viruses to inject malicious code into critical files that run the operating system or important programs. The computer virus is activated when the system boots or the program runs.

Multipartite Virus

These malicious programs spread across a network or other systems by copying themselves or injecting code into critical computer resources.

Macro Virus

Microsoft Office files can run macros that can be used to download additional malware or run malicious code. Macro viruses deliver a payload when the file is opened and the macro runs.

Computer viruses are standard programs; instead of offering useful resources, these programs can damage your device. Computer viruses are typically crafted by hackers with various intentions, like stealing sensitive data to causing chaos in systems. Some hackers create these malicious programs for fun or as a challenge, while others have more sinister motives like financial gain or cyber warfare.

Hackers may exploit weak points in an operating system or app to acquire unapproved access and power over a user’s machine to achieve their goals.

• Ego-driven: Some virus authors seek fame within the hacker community by creating destructive or widespread viruses that garner media attention.
• Cybercrime: Hackers often use computer viruses as tools for ransomware attacks, identity theft, and other forms of online fraud.
• Sabotage: In some cases, disgruntled employees create computer viruses to intentionally damage their employer’s infrastructure.
• Cyber espionage: State-sponsored hackers may develop advanced persistent threats (APTs) using custom-made malware designed for long-term infiltration into targeted networks.

For a threat actor to execute a virus on your machine, you must initiate execution. Sometimes, an attacker can execute malicious code through your browser or remotely from another network computer. Modern browsers have defenses against local machine code execution, but third-party software installed on the browser could have vulnerabilities that allow viruses to run locally.

The delivery of a computer virus can happen in several ways. One common method is via a phishing email . Another technique is hosting malware on a server that promises to provide a legitimate program. It can be delivered using macros or by injecting malicious code into legitimate software files.

At their core, computer viruses are discreet programs that hitch a ride on other files or applications. In most cases, their primary objective is to replicate and spread like wildfire.

Computer viruses function as malicious software programs designed to infect other programs by modifying them in some way. In doing so, a virus will attach itself to an unsuspecting file or application in order to spread.

The Infection Process

A virus can attach itself to any legitimate program or document that supports macros to execute its code, such as an email attachment or a file download from a website. Once the file is opened or downloaded, the virus springs into action and starts executing.

Hiding in Plain Sight

Computer viruses can be quite crafty to remain hidden from both users and antivirus software alike. Viruses employ stealth techniques such as polymorphism, which changes their appearance, or encryption methods.

The Damage Done

Once activated, a virus may wreak havoc on your computer system. It can steal sensitive data, corrupt files, slow down performance, and even crash your entire system. It can spread from system to system after a user takes action that either intentionally or accidentally facilitates it.

It’s important to note that viruses are just one type of malware, and many other types of malicious software can harm your computer or steal your personal information.

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Computer viruses spread through various channels, and being aware of these channels is essential to protect yourself and your organization from infection.

Email Attachments

One method of virus transmission is through email attachments. Hackers often disguise their malicious code as seemingly harmless files, such as documents or images unsuspecting users open without a second thought. For example, Ursnif banking Trojan campaigns are known to spread via email attachments posing as invoices or financial statements.

Internet Downloads

Viruses can also hide in software installers, media files, or even browser extensions that you download from the web. It’s important to be cautious when downloading files from unknown sources or sketchy websites. A notorious case was the Download.com scandal, where popular applications were bundled with adware and other unwanted programs by default.

File Sharing Networks

File sharing networks like torrent sites and peer-to-peer platforms can easily transmit viruses. Innocent-looking movie torrents or cracked software may carry hidden payloads designed to compromise your device upon installation. For example, The Pirate Bay used a browser-based cryptocurrency miner, so when someone visited the website, their computer was used to mine cryptocurrency without their knowledge or consent.

Removable Media

Viruses can attach to removable media, such as USB drives and CDs/DVDs, infecting any computer they’re plugged into. The infamous Stuxnet worm is a prime example of a virus that spreads through removable media.

To protect yourself and your organization from computer viruses, always exercise caution and employ robust cybersecurity measures like up-to-date antivirus software and regular system scans. Remember, knowledge is power, especially when preventing viruses and cyber-attacks.

A computer worm is a type of malware designed to replicate itself to spread to other computers. Unlike computer viruses, worms do not require a host program to spread and self-replicate. Instead, they often use a computer network to spread themselves, relying on security failures on the target computer to access it.

Once a worm infects a computer, it uses that device as a host to scan and infect other computers. When these new worm-infested computers are compromised, the worm continues to scan and infect other computers using these computers as hosts. Worms operate by consuming heavy memory and bandwidth loads, resulting in overloaded servers, systems, and networks.

The way a computer virus acts depends on how it’s coded. It could be something as simple as a prank that doesn’t cause any damage, or it could be sophisticated, leading to criminal activity and fraud. Many viruses only affect a local device, but others spread across a network environment to find other vulnerable hosts.

A computer virus that infects a host device continues delivering a payload until it’s removed. Most antivirus vendors offer small removal programs that eliminate the virus. Polymorphic viruses make removal difficult because they change their footprint consistently. The payload could be stealing data, destroying data, or interrupting services on the network or the local device.

While overlapping in intention and meaning, malware and viruses are two distinct terms that are often used interchangeably.

Malware is a general term for any type of malicious software, while a virus is a specific type of malware that self-replicates by inserting its code into other programs. While viruses are a type of malware, not all malware is a virus.

Malware can take many forms, including viruses, worms, trojans, spyware , adware, and ransomware, and it can be distributed through infected websites, flash drives, emails, and other means. A virus requires a host program to run and attaches itself to legitimate files and programs. It causes a host of malicious effects, such as deleting or encrypting files, modifying applications, or disabling system functions.

Malware authors write code that is undetectable until the payload is delivered. However, like any software program, bugs could present issues while the virus runs. Signs that you have a computer virus include:

• Popup windows, including ads (adware) or links to malicious websites.
• Your web browser home page changes, and you did not change it.
• Outbound emails to your contact list or people on your contact list alert you to strange messages sent by your account.
• The computer crashes often, runs out of memory with few active programs or displays the blue screen of death in Windows.
• Slow computer performance even when running few programs or the computer was recently booted.
• Unknown programs start when the computer boots or when you open specific programs.
• Passwords change without your knowledge or your interaction on the account.
• Frequent error messages arise with basic functions like opening or using programs.

The web contains millions of computer viruses, but only a few have gained popularity and infect record numbers of machines. Some examples of widespread computer viruses include:

• Morris Worm – One of the earliest and most pervasive computer virus examples, this self-replicating computer program spread through the early Internet in 1988, slowing down or crashing many machines.
• Nimda – This particular type of worm targeted web servers and computers running Microsoft Windows operating systems, spreading through multiple infection vectors in 2001.
• ILOVEYOU – A highly destructive worm that spread via email, disguised as a love confession and caused widespread damage in 2000 by overwriting files.
• SQL Slammer – A fast-spreading computer worm that exploited a vulnerability in Microsoft SQL Server, causing network congestion and disrupting Internet services in 2003.
• Stuxnet – A sophisticated worm designed to target and sabotage industrial control systems, particularly Iran’s nuclear program, by exploiting zero-day vulnerabilities in 2010.
• CryptoLocker – This ransomware Trojan, which infected hundreds of thousands of computers in 2013, encrypted victims’ files and demanded a ransom for their decryption.
• Conficker – Emerging in 2008, this worm exploited vulnerabilities in Windows operating systems, creating a massive botnet and causing widespread infection.
• Tinba – First discovered in 2012, this banking Trojan primarily targeted financial institutions, aiming to steal login credentials and banking information.
• Welchia – A worm that aimed to remove the Blaster worm from infected systems and patch the exploited vulnerability but caused unintended network congestion in 2003.
• Shlayer – A macOS-specific Trojan that primarily spreads through fake software updates and downloads, delivering adware and potentially unwanted programs since 2018.

Removing a computer virus can be a challenging task, but there are several steps you can take to get rid of it. Common steps to remove a computer virus include:

• Download and install antivirus software: Assuming you don’t already have antivirus software installed, download and install a real-time and on-demand solution, if possible. A real-time malware scanner scans for viruses in the background while you use the computer. You must start the on-demand scanner whenever you want to scan your device.
• Disconnect from the internet: Some computer viruses use the internet connection to spread, so it’s best to disconnect from the internet when removing a virus from your PC to prevent further damage.
• Delete any temporary files: Depending on the type of virus, deleting temporary files can also delete the virus, as some viruses are designed to initiate when your computer boots up.
• Reboot your computer into safe mode: To help mitigate damages to your computer while you remove a virus, reboot your device in ‘Safe Mode.’ This will inhibit the virus from running and allow you to remove it more effectively.
• Run a virus scan: Run a full scan using your antivirus software, opting for the most thorough or complete scanning option available. If possible, cover all your hard drive letters during the scan.
• Delete or quarantine the virus: Once the virus is detected, your antivirus software will give you the option to delete or quarantine the virus. Quarantining the virus will isolate it from the rest of your computer to prevent it from causing further damage.
• Reboot your computer: Assuming you’ve effectively removed the virus, your computer can be rebooted. Simply turn on the device as you would do so normally without initiating the “Safe Mode” option.
• Update your browser and operating system: To complete the virus removal process, update your operating system and web browser to the latest version possible. Browser and OS Updates often contain fixes for particular vulnerabilities and exploits.

Given the general nature of this process, the outcome may vary from virus to virus and device to device. If you are unsure if you’ve effectively removed a virus from your computer, contact an IT or computer professional for assistance.

Computer viruses can damage your PC, send sensitive data to attackers, and cause downtime until the system is repaired. You can avoid becoming the next computer virus victim by following a few best practices:

• Install antivirus software: Antivirus should run on any device connected to the network. It’s your first defense against viruses. Antivirus software stops malware executables from running on your local device.
• Don’t open executable email attachments: Many malware attacks including ransomware start with a malicious email attachment . Executable attachments should never be opened, and users should avoid running macros programmed into files such as Microsoft Word or Excel.
• Keep your operating system updated: Developers for all major operating systems release patches to remediate common bugs and security vulnerabilities. Always keep your operating system updated and stop using end-of-life versions (e.g., Windows 7 or Windows XP).
• Avoid questionable websites: Older browsers are vulnerable to exploits used when just browsing a website. You should always keep your browser updated with the latest patches and avoid these sites to prevent drive-by downloads or redirecting you to sites that host malware.
• Don’t use pirated software: Free pirated software might be tempting, but it’s often packaged with malware. Download vendor software only from the official source and avoid using software pirated and shared software.
• Use strong passwords: Make sure your passwords are highly secure and difficult to guess. Avoid using the same password across multiple accounts and change them regularly to mitigate vulnerabilities and prevent hackers from stealing them.
• Remain vigilant: Always be cautious when downloading files or software from the internet or opening suspicious email attachments. Turn off file sharing and never share access to your computer with someone you don’t know. Also, avoid keeping sensitive or private information stored on your computer

What Is a Computer Virus?

One of the oldest types of computer threats, viruses are nasty bits of malware that hijack your computer’s resources to replicate, spread, and cause all sorts of chaos. Keep reading to learn how viruses work and how you can protect your computer from viruses with common-sense tips and a dedicated cybersecurity tool.

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Computer viruses are just like that. If you’re unlucky enough to catch one (but don’t beat yourself too much if you do, because they are incredibly common), expect havoc to be wrecked on your hard disk — slower PC performance , damaged or destroyed files, and everything in between.

A computer virus definition, Wikipedia-style

Looking for an essay-friendly definition? Here it goes:

A computer virus is a program or piece of code designed to damage your computer by corrupting system files, wasting resources, destroying data or otherwise being a nuisance. 

Viruses are unique from other forms of malware in that they are self-replicating — capable of copying themselves across files or other computers without a user's consent.

Basically, they are really contagious.

Virus, malware, Trojan... what's the difference?

Not every piece of software that attacks your PC is a virus. Computer viruses are just one kind of malware (mal-icious soft- ware ). Here are some of the other, most common kinds:

Trojans : like the ancient wooden horse full of attackers it takes its name from, this malware pretends to be harmless legitimate software, or comes embedded in it, in order to trick the user and open up the gates for other malware to infect a PC.

Spyware : with examples such as keyloggers , this kind of malware is designed to spy on users, save their passwords, credit card details, other personal data and online behavior patterns, and send them off to whoever programmed it.

Worms : this malware type targets entire networks of devices, hopping from PC to PC.

Ransomware : this malware variety hijacks files (and sometimes an entire hard drive), encrypts them, and demands money from its victim in exchange for a decryption key (which may or may not work, but it probably won’t).

Adware : this exceedingly irritating kind of malware floods victims with unwanted ads, and opens up vulnerable security spots for other malware to wiggle its way in.

To recap, viruses are just one of several kinds of malware out there. Strictly speaking, Trojans, ransomware , etc, are not computer viruses, though many people use the shorthand “virus” to refer to malware in a general sense. Altough some devices like phones and iPads are not likely to get a virus , they are not immune to other threats.

Why do people make viruses, and what do they do?

Unlike the bio variety, computer viruses don’t just “happen”. They are manufactured, often with great care, intentionally targeting computers, systems and networks.

But what are these viruses used for?

Well, “fun”. Trolling by software, computer code graffiti… The earliest computer viruses were essentially programmers playing around, like the (maybe, probably) first one, known as the Creeper virus back in 1971, which displayed the message “I’m the creeper, catch me if you can!”.

Not quite Sylvia Plath

Or the Stoned virus , which randomly displayed the words “Your computer is stoned. Legalize marihuana!” on your screen (and stayed in stoner character throughout by doing absolutely nothing else).

Or a personal favorite: the virus that pretends to be a message from a well-known software company, offering you a free cup holder if you download and install it, at which point it opens your PC’s CD tray (remember CD trays?).

Sadly, not all viruses are so cuddly. Take it from Batman’s butler: some people just want to watch the world burn — and computer viruses are a very effective way to spread chaos far and wide.

Like the ILOVEYOU virus , which destroyed the files of more than 50 million internet users worldwide, rendered PCs unbootable, copied people’s passwords and sent them to its creators, and caused up to US$9 billion in damages in the year 2000.

Even that amount pales in comparison to the US$37 billion in damages caused by the Sobig.F virus , which stopped computer traffic in Washington DC and grounded Air Canada for a while.

And then there’s the Mydoom virus , which caused such cyber bloating that it’s believed to have slowed worldwide internet traffic by 10% the day of its release.

Yes, there is a tiny, tiny subset of “good” computer viruses — such as the Cruncher virus, which compresses every file that it infects and theoretically tries to help by saving precious hard disk space.

For example, there’s a virus out there labelled Linux. Wifatch which appears to do nothing other than keeping other viruses out of your router . Linux.Wifatch is itself a virus — it infects a device without its user’s consent and coordinates its actions through a peer-to-peer network — but instead of hurting you, it acts as a sort of security guard.

(But still, there are far better ways to secure your router — and even the creators of Linux.Wifatch tell you not to trust it).

Other “well-intentioned” viruses want to act like a vaccine in that they force people, corporations and governments to strengthen their safety measures and therefore become able to repel genuine threats.

Some virus creators argue they make the world safer by pointing out security gaps and flaws that can be exploited by other viruses with truly malicious intentions.

“What could possibly go wrong?”, asked the first ten minutes of every pandemic disaster movie ever made. The truth is that viruses quickly overwhelm the defenses they’re supposed to put to the test — take the Code Red virus , which in true disaster film fashion attacked the White House (OK the White House’s web server but still OMG) and caused 2.6 billion dollars in damage worldwide.

Some vaccine.

How do computer viruses spread?

Here are some common ways in which you can get infected with a computer virus:

Email viruses

Email is one of the favorite means of transportation for computer viruses everywhere. You can get computer viruses through email by:

Opening an attachment . Often named as something harmless (such as “ Your flight itinerary ”), an executable program file (.com, .exe, .zip, .dll, .pif, .vbs, .js, .scr) or macro file type (.doc, .dot, .xls, .xlt, xlsm, .xsltm…).

Opening an email with an infected body. In these days of rich graphics and colors and bells and whistles, some viruses are being transported in the HTML body of the email itself. Many email services disable HTML by default until you confirm you trust the sender.

Instant messaging viruses

Instant messaging (IM) is another means for viruses to spread. Skype, Facebook Messenger, Windows Live Messenger and other IM services are inadvertently used to spread viruses to your contacts with infected links sent through chat messages.

These instant messaging and social media viruses spread wide and fast because it’s far easier to get people to click on a link when it’s delivered in a message coming from someone they trust, as opposed to a an email from a stranger.

File sharing viruses

Peer-to-peer file sharing services like Dropbox, SharePoint or ShareFile can be used to propagate viruses too. These services sync files and folders to any computer linked to a specific account, so when someone (inadvertently or otherwise) uploads a virus-infected file to a file-sharing account, that virus gets downloaded to everyone else with access to that shared folder.

Some file sharing services, such as Google Drive , scan uploaded files for viruses (although it only scans files smaller than 25MB, giving virus spreaders an easy out — they just have to make sure their virus-infected files are larger than that).

But most other services do not scan for viruses at all, so it’s your responsibility to make sure that you’re protected against any potential threats contained in the file they’re downloading.

Software download viruses

Fake antivirus infections are one of the most common types of virus-loaded software downloads. Scammers and cyber criminals use aggressive pop-ups and ads to scare users into believing that a non-existent virus has been detected in their PC, and compels them to download their “antivirus” software in order to clear the threat.

Instead of ridding the computer of viruses, this fake antivirus proceeds to infect the PC with malware, often with devastating consequences for the victim’s files, hard drive, and personal information.

Unpatched vulnerable software

Last but not least, one of the most common (yet most often overlooked) means for viruses to spread is unpatched software.

Unpatched software refers to software and apps which have not been updated with the latest security updates from the developer, in order to plug up security holes in the software itself.

Unpatched software is a major cybersecurity headache for businesses and organizations, but with criminals exploiting vulnerabilities in outdated versions of such popular programs as Adobe Reader, Java, Microsoft Windows or Microsoft Office , us civilians are very much at risk of infection too.

Types of computer virus

Here’s a list of different types of computer viruses currently out there:

Boot Sector Virus

The boot sector is the part of your PC’s hard drive that loads your computer’s operating system — such as Microsoft Windows. A boot sector virus infects the master boot record (MBR), so the virus loads onto the computer memory during startup.

Boot sector viruses used to be propagated mainly via pluggable devices, like USB keys, floppy disks and CD-ROMS. As technology moves on, boot sector viruses have become much rarer, and these days they mostly live on as email attachments.

Examples of boot sector viruses:

Elk Cloner: this early 1980s virus was attached to a game. At the 50th time the game was started, the virus displayed a poem on-screen.

Stoned: the initial variety displayed on-screen messages in favor of the legalization of marihuana. Its signature (though not the virus itself) crept up into the bitcoin blockchain in 2014.

Parity Boot: another “vintage” virus, this one was the most prevalent virus in Germany up to 1996.

Brain: considered to be the first computer virus for MS-DOS, it was created by the Pakistani Alvi brothers as an attempt to protect their medical software from copyright infringement — an attempt that quickly got out of hand, much to their chagrin.

Michelangelo: every year on March 6 (the birthday of artist Michelangelo), this virus would come alive and overwrite the first 100 sectors of a hard drive with nulls, making it impossible for everyday users to retrieve their files.

Direct Action Virus

These viruses are designed to “pass through” your computer: they get in, generally spread around files of a specific type (COM or EXE files, generally), and when they are done, they delete themselves. They are the most common type of virus out there and the easiest to create — which also makes them the simplest to get rid of.

Examples of direct action viruses:

Win64.Rugrat: also known as the Rugrat virus, this early example of direct action virus could would infect all 64-bit executables it could find in the directory and subdirectories in which it was launched.

Vienna virus: the Vienna virus has the distinction of being the first virus to be destroyed by an antivirus. It searches for .com files and destroys some of them while attempting to infect them.

Resident Virus

Unlike the direct action viruses we mentioned before, memory resident viruses actually set up camp in your computer’s primary memory (RAM). This is bad news, because they can keep working even after you’ve rid yourself of the original infector. Some act fast, some do their damage slowly — and are therefore harder to detect.

Examples of memory resident viruses:

Jerusalem virus (a.k.a. Friday 13th virus): after finding its way into your RAM and hiding inside it, this virus would delete programs from your computer on Friday 13, or increase the size of infected programs until they were too big to run.

Onehalf virus: sometimes known as the Freelove virus, or the Slovak Bomber, this virus slowly encrypts its way through your hard disk. Once it’s done with half (and on the 4th, 8th, 10th, 14th, 18th, 20th, 24th, 28th and 30th day of any month), it displays the message “Dis is one half. Press any key to continue…”

Magistr virus: this very destructive virus emails itself to your contact list, deletes every other file, wrecks your CMOS and BIOS — and leaves you insulting messages to boot.

Multipartite Virus

These ultra-versatile viruses double their spreading power by targeting both your files and your boot space. That way, even after you’ve succeeded in removing all the infected files in your computer, the virus still lingers hidden in the boot sector, ready to strike again — and if you clean the boot sector, the virus will re-infect it by jumping from one of the infected files.

Examples of multipartite viruses:

Junkie virus: this multipartite virus was transmitted in a file called HV-PSPTC.ZIP., supposedly a Pacific Strike computer game. Which it wasn’t.

Tequila virus: this one avoids files which contain the letters “v” and “sc” in their name, and likes to display the message “BEER and TEQUILA forever!”

• Shhhh, don't argue with the virus

Invader virus: this one starts off nicely by playing a Mozart tune, but the moment you hit CTRL+ALT+DEL to reboot, it overwrites the first line of your hard disk with a copy of the virus.

Polymorphic Virus

The mutants of the computer virus world, these viruses shape-shift in order to avoid detection, while holding onto their basic threat capabilities. After infecting your files, these viruses replicate themselves in a slightly different way — which makes them very difficult to fully detect and remove.

Examples of polymorphic viruses:

Satanbug virus: despite its let’s face it quite badass name, this polymorphic virus doesn’t intentionally damage your files — but with its up to nine levels of encryption, virus scanners have a hard time removing it from your PC.

VirLock virus: part ransomware, part polymorphic virus, the Win32/VirLock virus encrypts your files and asks you for ransom — but it also changes shape every time it spreads.

Macro Virus

Macro viruses are written in macro language, with the intention of embedding them within software that allows macro mini-programs such as Microsoft Word. That means your PC can be infected by Word Document viruses.

Examples of macro viruses:

Melissa: distributed through email attachments, once this virus infects your PC it makes its way to your Microsoft Outlook mail client and mails itself to the first 50 contacts in your address book, potentially slowing down or even completely disabling servers in a chain reaction of suck.

How do I protect myself against viruses?

By now we have established you definitely don’t want any of these viruses anywhere near your files, your hard drive or your network. Now, let’s look at how you can avoid computer viruses in the first place.

Use antivirus protection

You have to, have to, have to have some form of antivirus software installed in your PC and your smartphone.

An antivirus is your first line of defense against viruses and a whole bunch of other malware that you seriously don’t want to have to deal with. If you think viruses are bad (and they are), there’s stuff out there that’s even worse. 

No excuses. Don’t want to fork out money? There are a few exceptionally good free antivirus software choices out there (ahem). Worried the software will slow down your PC? Ours is so light, you won’t even notice it’s there.

If you’re looking for something extra, premium antivirus programs can offer all kinds of added security features - like our Ransomware Protection shield, which stops anyone from hijacking your files and extorting money from you. For instance, we help you find your phone if you lose it, which is pretty neat.

But hey, you don’t have to get our antivirus (even though it’s free and awesome). Just get some antivirus.*

*although (AND THIS IS THE LAST THING WE’LL SAY ABOUT THIS WE PROMISE) you really shouldn’t get just any free antivirus you find laying around out there, because some of them are actually malware carriers in disguise, and some others are just really blah. So for your own good, always go with an antivirus from a cyber security company whose reputation is at least as good as ours.

Use that pretty head of yours

Apart from letting your antivirus detect and remove viruses, you’ll be doing yourself a huge favor by using proper cyber hygiene in the first place and following some basic internet safety tips:

Don’t just click on any link your friends send you on social media — especially if the message is just a link with no context, or if the words in the message don’t quite sound like them. People’s Facebook accounts get hacked and used to spread out viruses and malware. When in doubt, message your friend directly and ask if they really meant to send you that link. Often, the answer will be “What!? No!”

Don’t open any email attachment unless you 100% know what it is. Cyber criminals often rely on your natural curiosity to spread viruses - they tell you you’ve won something, but you haven’t entered any contests; or they send you a “flight itinerary”, but you aren’t planning on going anywhere. So you open the attachment to see what it’s all about, and bam, you’re infected. So, don’t.

Don’t fall for “Your PC is infected!” hysteric messages and pop-ups that aren’t coming directly from your antivirus. There is a very good chance they’re trying to lure you into downloading a fake antivirus and take your money, infect you computer with malware, or both. When our antivirus catches something, we let you know with a satisfying little message, and that’s it. We don’t ask you to download anything else, or pay any money.

Don’t enable macros in Microsoft Office . A few years ago we would have recommend you to disable macros, but Microsoft already does that by default. Which means cyber criminals try to trick you into enabling them with all sorts of dark mind tricks and fake warnings when you receive an infected email. Don’t fall for them.

But seriously. Antivirus. Now.

How do I remove a virus from my computer?

Stopping a virus from getting into your PC is a lot easier than deleting a virus that’s already in your computer, but if you suspect your PC is already infected with a virus, do not fret. We’ve got you covered.

Is your computer infected?

If your computer has become very slow all of a sudden for no particular reason; if you’re being flooded with pop-up messages out of the blue; if programs and apps start by themselves, and you can hear the hum-hum of your hard drive constantly working in the background…

… it may be.

Here’s how you delete a virus

We have a step-by-step guide that tells you how to get rid of computer viruses the safe and easy way. Give it a read. Or, if your phone has been acting oddly lately, check out our detailed guide to learn how to tell if your phone has a virus or other form of nasty malware — and how to get rid of it.

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• Government Exam Articles

Computer Virus and its Types

A computer virus is a kind of malicious computer program, which when executed, replicates itself and inserts its own code. When the replication is done, this code infects the other files and program present on your system. 

These computer viruses are present in various types and each of them can infect a device in a different manner. 

In this article, we shall discuss in detail what is a computer virus and what are its different types. Also, we will read on to know what is an Anti-virus and how it can nullify a virus in our computer devices, along with some sample questions from the competitive exam point of view. 

To know more about the Fundamentals of Computer , visit the linked article. 

Apart from being aware of what a computer virus is, this topic is even important for candidates preparing for Government exams. Major competitive exams in the country comprise Computer Knowledge as an integral part of the syllabus and questions based on virus and anti-virus can also be expected in these exams. 

Thus, to excel in the upcoming Govt exams, aspirants must go through this article in detail and carefully study the different types of viruses.

What is a Computer Virus?

A computer virus is a program which can harm our device and files and infect them for no further use. When a virus program is executed, it replicates itself by modifying other computer programs and instead enters its own coding. This code infects a file or program and if it spreads massively, it may ultimately result in crashing of the device. 

Across the world, Computer viruses are a great issue of concern as they can cause billions of dollars’ worth harm to the economy each year. 

Since the computer virus only hits the programming of the device, it is not visible. But there are certain indications which can help you analyse that a device is virus-hit. Given below are such signs which may help you identify computer viruses:

• Speed of the System – In case a virus is completely executed into your device, the time taken to open applications may become longer and the entire system processing may start working slowly
• Pop-up Windows – One may start getting too many pop up windows on their screen which may be virus affected and harm the device even more
• Self Execution of Programs – Files or applications may start opening in the background of the system by themselves and you may not even know about them
• Log out from Accounts – In case of a virus attack, the probability of accounts getting hacked increase and password protected sites may also get hacked and you might get logged out from all of them
• Crashing of the Device – In most cases, if the virus spreads in maximum files and programs, there are chances that the entire device may crash and stop working

The first thing which you might notice in case of virus attack is the speed with which your system shall process. And then gradually other changes can also be observed. 

Types of Computer Virus

Discussed below are the different types of computer viruses:

• Boot Sector Virus – It is a type of virus that infects the boot sector of floppy disks or the Master Boot Record (MBR) of hard disks. The Boot sector comprises all the files which are required to start the Operating system of the computer. The virus either overwrites the existing program or copies itself to another part of the disk.
• Direct Action Virus – When a virus attaches itself directly to a .exe or .com file and enters the device while its execution is called a Direct Action Virus. If it gets installed in the memory, it keeps itself hidden. It is also known as Non-Resident Virus.
• Resident Virus – A virus which saves itself in the memory of the computer and then infects other files and programs when its originating program is no longer working. This virus can easily infect other files because it is hidden in the memory and is hard to be removed from the system.
• Multipartite Virus – A virus which can attack both, the boot sector and the executable files of an already infected computer is called a multipartite virus. If a multipartite virus attacks your system, you are at risk of cyber threat.
• Overwrite Virus – One of the most harmful viruses, the overwrite virus can completely remove the existing program and replace it with the malicious code by overwriting it. Gradually it can completely replace the host’s programming code with the harmful code.
• Polymorphic Virus – Spread through spam and infected websites, the polymorphic virus are file infectors which are complex and are tough to detect. They create a modified or morphed version of the existing program and infect the system and retain the original code.
• File Infector Virus – As the name suggests, it first infects a single file and then later spreads itself to other executable files and programs. The main source of this virus are games and word processors.
• Spacefiller Virus – It is a rare type of virus which fills in the empty spaces of a file with viruses. It is known as cavity virus. It will neither affect the size of the file nor can be detected easily.
• Macro Virus – A virus written in the same macro language as used in the software program and infects the computer if a word processor file is opened. Mainly the source of such viruses is via emails.

Government exam aspirants can check the links given below for the detailed section-wise syllabus for the other subjects apart from Computer Awareness:

How To Protect Your Computer from Virus?

The most suitable way of making your computer virus-free is by installing an Anti-virus software. Such software help in removing the viruses from the device and can be installed in a computer via two means:

• Online download
• Buying an Anti-virus software and installing it

Further below, we bring to you details as to what anti-virus is and what are its different types along with a few examples.

Moving further, candidates can also refer to the following links to learn more about Computer Knowledge and prepare themselves accordingly:

• Difference Between RAM and ROM
• Difference Between MS Excel and MS Word
• Difference Between IPV4 and IPV 6
• Difference Between Firewall and Antivirus
• Difference Between WWW and Internet
• Difference Between Notepad and WordPad
• Difference Between Virus and Malware

What is an Anti-Virus?

An anti-virus is a software which comprises programs or set of programs which can detect and remove all the harmful and malicious software from your device. This anti-virus software is designed in a manner that they can search through the files in a computer and determine the files which are heavy or mildly infected by a virus. 

Given below is a list of few of the major antivirus software which is most commonly used:

• Norton Antivirus
• F-Secure Antivirus
• Kaspersky Antivirus
• AVAST Antivirus
• Comodo Antivirus
• McAfee Antivirus

These are few of the many anti-virus software widely used to remove viruses from a device. 

Sample Questions on Computer Virus and Anti-Virus

As discussed above, Computer Awareness is a common topic for major Government exams and questions based on Computer Virus and Antivirus may also be asked in the exam.

Thus, given below are a few sample computer virus questions and answers for the assistance of aspirants. 

Q 1. Which of the following is not a type of computer virus?

• Polymorphic virus
• Space filler virus
• Multipartite virus
• Boot sector virus

Answer: (4) Trojan

Q 2. Which of these was the first computer virus?

• Crypto Locker
• Morris Worm

Answer: (1) Creeper

Solution: Creeper was the first-ever computer virus and was an experimental self-replicating virus released in the year 1971.

Q 3. Which of the following is not a source of the virus entering your system?

• All of the above
• None of the above

Answer: (5) All of the above

Q 4. The other name for Non-Resident virus is _________

• Direct Action Virus
• Boot Sector Virus
• Multipartite Virus
• Overwrite Virus
• Polymorphic Virus

Answer: (1) Direct Action Virus

Q 5. Which of the following viruses is also known as “Cavity Virus”?

• Space Filler Virus

Answer: (2) Space Filler Virus

For more questions to ace the upcoming competitive exams, aspirants can visit the articles given below:

• Free Online Government Exam Quiz
• Free Online Mock Test Series with Solutions
• Previous Year Govt Exam Question Papers PDF with Solutions

Also, to get the best Preparation Strategy for Competitive exams , candidates can visit the linked article. 

Get the latest exam information and study material at BYJU’S and keep yourself updated. 

Frequently Asked Questions on Computer Virus and its Types

Q 1. what is the definition of a computer virus, q 2. what are the main computer viruses.

Ans. The main types of computer virus are as follows:

• Resident Virus
• File Infector Virus

Q 3. What is the Creeper Virus?

Q 4. what are the examples of a computer virus.

Ans. Given below are a few examples of a computer virus:

• CryptoLocker

Q 5. How to secure a computer system from a virus attack?

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Optogenetics, the Big Revolution in Brain Study

Intellectual abilities of artificial intelligence, openmind books, scientific anniversaries, edward o. wilson and island biodiversity, featured author, latest book, the history of computer viruses.

On November 10, 1983, a handful of seminar attendees at Lehigh University, Pennsylvania, USA, heard for the first time the term “virus” applied to computing. The use of the word was strange. The virus that was then on everyone’s mind was the one isolated a few months earlier at the Pasteur Institute in Paris that could be the cause of a new disease called AIDS. In the digital world, talking about viruses was almost nonsense. The first PC had been launched on the market just two years earlier and only the most technologically informed were running an Apple II computer or one of its early competitors.

However, when on that day the graduate student from the University of Southern California Fred Cohen inserted a diskette into a VAX11/750 mainframe computer, the attendees noted how code hidden in a Unix program installed itself and took control in a few minutes , replicating and spreading to other connected machines, similar to a biological virus.

Cohen tells OpenMind that it was on November 3 when a conversation with his supervisor, Leonard Adleman, led to the idea of ​​giving the name of virus to that code capable of infecting a network of connected computers. The Cohen virus was simple: “The code for reproduction was perhaps a few lines and took a few minutes to write,” says the author. “The instrumentation and controls took almost a day.”

Cohen published his creation in 1984, in an article that began: “This paper defines a major computer security problem called a virus.” But though the extensive research of Cohen and Adleman in the specialized literature would draw attention to their existence, the truth is that before that first virus defined as such appeared, there had already been earlier cases.

Interactive timeline: A malware history

[+] Full screen

Catch me if you can

In 1971, Robert Thomas, from the company BBN, created Creeper , a program that moved between computers connected to ARPANET and that displayed the message “I’m the creeper: catch me if you can.” According to David Harley, IT security consultant and researcher for the ESET company, “in the research community, we usually consider the experimental program Creeper to be the first virus and/or worm.”

Moreover, a year before Cohen’s seminar, 15-year-old Rich Skrenta developed Elk Cloner, the first computer virus—not named that yet—that spread outside a laboratory. Skrenta created it as a joke for his friends, whose Apple II computers became infected by inserting a diskette with a game that hid the virus.

So, Cohen was not really the first one. But according what computer security expert Robert Slade explains to OpenMind, the special thing in Cohen’s case was not so much his programming as his method. “He was doing the original academic research on the concept; his structure of antiviral software is still comprehensive despite all the developments since.” Cohen also introduced an informal definition of virus: “a program that can infect other programs by modifying them to include a, possibly evolved, version of itself.”

Those first viruses were technological demonstrations. The motivation of their creators was research and their codes were not malicious. Cohen points out that the objective of his program was “to measure spread time, not to attack.” In the case of Creeper , it was about designing a mobile application that could move to the machine where the data resided, instead of going the other way. As the professor of Computer Science at the University of Calgary (Canada) John Aycock points out to OpenMind, computer viruses were born as “a natural product of human curiosity.” And as such, “their invention was inevitable.”

The first malicious codes

It was also inevitable that the first malicious codes would soon emerge. In 1986, Brain appeared, a virus created by two Pakistani brothers whose purpose was to punish the users of IBM computers who installed a pirated copy of software developed by them. However, the effects of Brain were slight and the virus included the contact information of its authors so that those affected could contact them and request a cure. Spread by means of diskettes, Brain reached international diffusion, giving rise to the birth of the first antivirus companies.

At the end of the 1980s, codes began to proliferate that erased data or disabled systems. In 1988, the worm created by Robert Morris infected many of the computers connected to the then nascent Internet, especially in research institutions, causing a drop in email services. Its effects were more damaging than anticipated by Morris himself, who became the first person to be prosecuted in the US under the Computer Fraud and Abuse Act of 1986.

In this way, so-called malware began to diversify into different families: worms are programs that move from one computer to another without hiding in another application, while Trojans are harmful programs with an innocent appearance. In 1995, WM/Concept appeared, which infected Word documents. “It opened the door for a plague of document-borne malware that dominated the threat landscape for several years after,” says Harley. The expert lists other typologies that have emerged over time, such as bots that manipulate other people’s systems to launch spam campaigns, send malware or denial of service attacks; or ransomware , codes that hijack a system and force the payment of a ransom, such as the recent case of WannaCry , which in May 2017 infected hundreds of thousands of computers in more than 150 countries.

To this threat landscape we must add the current media, such as social networks, which facilitate the expansion of malware. As explained to OpenMind by Jussi Parikka, expert in technological culture at the Winchester School of Art of the University of Southampton (United Kingdom) and author of Digital Contagions: A Media Archeology of Computer Viruses (2nd ed., Peter Lang Publishing, 2016), “the online platforms for communication and interaction are themselves part of the problem due to their various security issues.”

But despite the many headaches caused by the malware, experts point out that these developments can benefit other technologies. Cohen argues that “benevolent” viruses can, for example, be useful in maintaining and updating systems. “I think artificial life (reproducing programs) still have enormous potential, largely unrealized as of today,” he reflects. “History will tell, but I still hold hope that viral computation will be a benefit to humanity in the future.”

Javier Yanes

More publications related to this article, more about technology, artificial intelligence, digital world, visionaries, more publications about ventana al conocimiento (knowledge window), comments on this publication.

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What is a Computer Virus?

A computer virus is a type of malicious software program (“ malware “) that, when executed, replicates itself by modifying other computer programs and inserting its code. When this replication succeeds , the affected areas are then said to be “ infected “. Viruses can spread to other computers and files when the software or documents they are attached to are transferred from one computer to another using a network , a disk , file-sharing methods , or through infected email attachments.

A computer virus is a type of harmful program . When it runs, it makes copies of itself and adds its code to other programs and files on your computer. These viruses come in different types , and each type can affect your device differently . Simply put, a computer virus changes how your computer works and aims to spread to other computers. It does this by attaching itself to normal programs or documents that can run code, known as macros .

What Does a Computer Virus Do?

A virus can harm or destroy data , slow down system resources , and log keystrokes , among other things. A virus can have unexpected or harmful outcomes during this procedure, such as destroying system software by corrupting data. Some viruses are made to mess things up by deleting files , messing up programs , or even wiping out your hard drive completely. Even if they’re not super harmful, viruses can still slow down your computer a lot, using up memory and making it crash often . Others might just make copies of themselves or send so much stuff over the internet that it’s hard to do anything online.

Virus vs. Malware – What is the difference? 

Viruses and malware are often used interchangeably, but they’re not quite the same. Here’s how they differ:

History of Computer Virus

Viruses have been attacking various devices for a long time, spreading through the Internet or other means. They are often created to steal information or completely ruin devices. The first computer virus, called the “ Creeper system ,” appeared in 1971 as an experimental virus that could copy itself. Following that, in the mid-1970s , the “ Rabbit ” virus emerged , which replicated very quickly and caused significant damage at the same pace. The virus known as “ Elk Cloner ” was created in 1982 by Rich Skrenta . It spread through a floppy disk containing a game and attached itself to the Apple II operating system.

The first virus for MS-DOS , called “ Brain ,” appeared in 1986 . It was designed by two Pakistani brothers and overwrote the boot sector of floppy disks , making it impossible for the computer to start. It was originally meant to be a copy protection system. In 1988 , more destructive viruses began to surface. Until then, most viruses were considered pranks with funny names and messages. However, in 1988, “ The Morris ” became the first widely spreading virus.

How To Prevent Your Computer From Viruses?

Keeping your computer safe from viruses is a lot like keeping yourself from catching a cold. Just as you might wash your hands regularly or avoid sick friends, there are simple steps you can take to protect your computer. Here are some easy tips:

1. Install Antivirus Software: Think of antivirus software as your computer’s doctor. It works around the clock to detect and block viruses before they can infect your system. Make sure to keep it updated!

2. Update Regularly: Keep your operating system , software , and apps up to date . Updates often include fixes for security vulnerabilities that viruses could exploit.

3. Be Cautious with Emails and Downloads: Don’t open emails or download attachments from unknown sources. If an email looks suspicious , even if you know the sender, it’s best to delete it.

4. Use Strong Passwords: Protect your accounts with strong , unique passwords . Consider using a password manager to keep track of them all.

5. Backup Your Data: Regularly back up your data to an external drive or cloud storage . If a virus does slip through , you won’t lose everything.

By following these steps, you can help keep your computer virus-free and running smoothly.

How To Remove Computer Viruses?

To remove a computer infection, you can choose from two options:

Do-it-yourself manual approach: This means you try to fix the problem on your own. Usually, you start by searching online for solutions. Then, you might have to do a lot of tasks to clean up your computer . It can take time and might need some experience to finish everything.

Get help from a reliable antivirus product: Another option is to use antivirus software . This software is designed to find and remove viruses from your computer. You just need to install it and let it do its job.

What is Antivirus?

Antivirus software is a program that searches for, detects , prevents , and removes software infections that can harm your computer. Antivirus can also detect and remove other dangerous software such as worms , adware , and other dangers . This software is intended to be used as a preventative measure against cyber dangers , keeping them from entering your computer and causing problems . Antivirus is available for free as well. Anti-virus software that is available for free only provides limited virus protection, whereas premium anti-virus software offers more effective security. For example Avast , Kaspersky , etc.

Also Check :

Anti-Virus | Its Benefits and Drawbacks How an Antivirus Works?

Different Types of Computer Virus

Each type has a unique way of infecting and damaging computers. Here are a few examples:

How do computer viruses spread?

Through the following activities you may get your device infected by the virus :

1. Sharing the data like music , files , and images with each other.

2. If you open a spam email or an attachment in an email that is sent by an unknown person.

3. Downloading the free game s, toolbars , media players, etc.

4. Visiting a malicious website .

5. Installing pirated software (s) etc.

Examples of Computer Viruses

A computer virus is a type of software designed to spread from one computer to another, similar to how a cold spreads between people. Just like a cold virus can make us sick, a computer virus can harm a computer’s performance and security . Here are some common examples of computer viruses:

These examples show how diverse computer viruses can be in their methods of infection and damage. Knowing about them can help you understand the importance of having reliable antivirus software and practicing safe browsing habits.

In conclusion, understanding what a computer virus is and recognizing the dangers it poses is crucial for keeping your data safe. These viruses are designed to infect , replicate , and damage the functioning of computers. Protecting your computer with antivirus software , being cautious with email attachments , and avoiding suspicious websites are essential steps to prevent virus infections. By staying informed and attentive , you can help safeguard your computer from the potential destruction caused by computer viruses.

What is a Computer Virus? – FAQs

How can you protect your computer system from viruses.

We can use antivirus software to keep your computer safe from viruses. Antivirus software works by comparing the files and programs on your computer to a database of known malware types. It will also monitor computers for the presence of new or undiscovered malware threats , as hackers are constantly generating and propagating new viruses. 

What are computer virus infection sources. 

The sources via which you can infect your system with viruses are : 1. Downloading programs/software from the internet. 2. Emails 3. External devices like pen-drives 4. Using an unknown CD to Boot data 5. Bluetooth / infrared

Computer viruses are typically propagated by email, file sharing, or CDs or by downloading file(s) from unauthenticated sources. 

What is an Anti-Virus?

An anti-virus is a piece of software that consists of programs or a collection of programs that can detect and remove all unsafe and malicious software from your system. This anti-virus software is created in such a way that it can look through a computer’s files and d etect which files are heavily or moderately infected by a virus.

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Detection and Diagnosis of Viral Infections

Diagnostic tests are paramount in determining the etiology of viral infections. Direct diagnostic methods assay for the presence of the virus, while indirect methods test for effects of the virus. Cell culture is the process of growing cells or tissues in the laboratory. Cell lines can be infected with patient samples to allow viral replication within the cells; observable cytopathic effects can help to identify the identity of the virus. Infected cells can also be used for immunofluorescence assays, which use fluorescently labeled virus-specific antibodies to identify viruses in fixed cells or tissues. A variety of diagnostic immunoassays exist, including enzyme-linked immunosorbent assays/enzyme immunoassays, western blots, lateral flow immunoassays, and agglutination reactions. Assays that detect viral nucleic acids are based upon the principles of PCR or nucleic acid hybridization, are extremely sensitive, and are specific for a particular virus.

Viruses have evolved alongside humans for as long as both have existed. “Filterable viruses” were classified as a separate pathogen upon their identification in 1898, but infectious diseases have been characterized throughout history by the clinical conditions they have caused, despite that the doctors of the time had no idea of the existence of microscopic pathogens. Hippocrates (460–377 BC), the father of modern medicine, described several illnesses characteristic of viral diseases, including influenza and poliomyelitis. The ability to definitively identify a specific virus as the cause of an illness has only become possible within the last 100 years.

As described in Chapter 1, “The World of Viruses,” bacteria can be viewed under a light microscope, but viruses are too small to be visualized with light microscopes. Consequently, the first efforts to identify specific viruses relied upon serology , the analysis of the protein antibodies found in blood that the immune system synthesizes against pathogens. Tissue culture , the ability to grow tissues and cells outside of a living organism in a controlled environment, was invented and refined in the first half of the 20th century. This led to the propagation of viruses using cell culture and the detection of the pathogenic effects that viruses exert upon cells. Both serology and tissue culture have been refined and are still vital techniques for the diagnosis of viral infections. Advances in molecular biology have also accelerated our ability to conclusively identify a virus.

The detection of a virus as the cause of an illness is important for many reasons. Several viral infections result in similar clinical symptoms, and viruses with serious effects need to be identified early in order to prescribe the best treatment. Likewise, the infection of high-risk groups, such as transplant recipients, pregnant women, or immunocompromised individuals, needs to be monitored so that critical sequelae can be addressed. The development and availability of antiviral drugs is increasing, and proper diagnosis ensures an effective treatment is prescribed. The typing of viruses is also effective in determining subtypes or strains of viruses, including those that are resistant to certain drugs or are more likely to cause cancer. In the field of epidemiology, most case definitions rely upon laboratory confirmation of the specific virus to confirm a case. Proper diagnosis ensures that accurate surveillance takes place and adequate control measures are instituted during epidemics. It also ensures the safety of transplanted human tissues and safeguards the blood supply. This chapter discusses commonly used techniques for the detection and diagnosis of viruses in clinical samples. Many of these methods are staples in virology research laboratories, as well.

7.1. Collection and Transport of Clinical Specimens

For identifying a specific virus, the type of specimen obtained depends upon the type of virus. The specimen will be isolated from the location of infection for viruses that establish localized infections ( Table 7.1 ). For instance, influenza virus is readily detected from nasopharyngeal swabs, and herpes simplex viruses can be isolated from the oral or genital lesions that these viruses cause ( Table 7.2 ). Viruses that establish systemic infection may be isolated from several different sources, depending upon the virus. The site of pathology is often a good place to start, although the virus may be present in the blood as well. For example, hepatitis B virus and hepatitis C virus infect hepatocytes (liver cells) but are detectable in serum.

Types of Specimens Collected for Viral Diagnosis

Specimens Collected for Select Human Viruses

The choice of diagnostic test will also depend upon the stage of infection. A person’s viral load is highest during acute infection but may drop to undetectable levels as the infection is cleared. On the other hand, it takes weeks for antibodies to develop during the primary response against a virus ( Fig. 7.1 ). As described in Chapter 6, “The Immune Response to Viruses,” IgM is the antibody isotype that is first produced by plasma cells against a pathogen, but the higher-affinity IgG isotype begins being secreted by plasma cells later during infection and during secondary responses. Therefore, the choice of diagnostic test will depend upon the patient’s stage of infection. Tests for the virus itself are best performed before or while symptoms are present. The levels of IgM versus IgG antibodies against a virus can be used to help determine if the infection recently occurred, but neither of these will be present at the beginning of a primary infection.

Considerations for choice of diagnostics.

Diagnostic tests are available that test directly for the presence of virus, which occurs during active infection, or antiviral antibodies, which take weeks to develop and continue for months following infection. IgM is produced during a primary response, while IgG takes additional time to develop and will be produced during secondary responses. The choice of test depends upon the state of the infection and whether virus or antibody is likely to be present at that time.

Care must be taken in the collection, storage, and transport of clinical specimens. Blood is collected into appropriate tubes, depending upon whether cells, serum, or plasma is required for the diagnostic test. Tubes containing sodium heparin or EDTA as an anticoagulant block the clotting of blood and are used to obtain white blood cells ( leukocytes) or plasma , the liquid fraction that remains when blood is centrifuged to pellet blood cells. Serum is obtained by allowing blood to clot and then centrifuging the clot, leaving behind the liquid portion of the blood ( Fig. 7.2 ). The difference between plasma and serum, therefore, is that plasma contains clotting factors that are part of the clot when serum is obtained. Antibodies and virus/virus antigen will be found in serum and plasma, although virus could also be found in the leukocytes, if they are a target of the virus. For collecting fluid from skin lesions, a sterile swab is used to collect the fluid and cells from a lesion that has been opened, and then the swab is placed in a special transport medium. The same transport medium is used for nasopharyngeal swabs, cervical swabs, rectal swabs, or throat swabs. Stool is collected into a clean, leak-proof container.

The constituents of blood.

Different diagnostic tests require different components of blood. (A) In a tube without anticoagulants, a clot will form that can be centrifuged to the bottom of the tube. The acellular liquid fraction is serum. A tube with heparin or ETDA as coagulants prevents clotting factors from working. The cellular fraction is centrifuged, and the liquid fraction—with clotting factors—is known as plasma. If unclotted blood is centrifuged slowly in a special medium to separate the blood by density, a thin white line of leukocytes, called the buffy coat, separates the plasma and red blood cell layers. In this case, about 55% of the total blood is plasma, 45% are erythrocytes, and <1% are leukocytes. Antibodies are found in serum or plasma. (B) A researcher examines two blood density centrifugations. Note the thin layer of leukocytes in between the plasma and red blood cell layers.

The susceptibility of viruses to environmental factors can be an issue for diagnostic tests that rely upon “live” virus. As mentioned in Chapter 5, “Virus Transmission and Epidemiology,” most viruses will become noninfectious after being exposed to extended periods of heat. Although nucleic acids may be able to be recovered from these samples, infectious virus will not be present. To prevent virus inactivation, samples other than blood that must be transported to diagnostic laboratories are refrigerated during shipment, or frozen at −80°C and shipped on dry ice if the transport will take 3 days or more. However, some viruses are not stable when frozen, including varicella zoster virus, respiratory syncytial virus, measles, and human cytomegalovirus. These viruses must be frozen in a special transport medium to prevent their inactivation. Much emphasis is placed upon the efficacy of the diagnostic test itself, but no assay can provide meaningful results if the specimen has not been collected, stored, and transported with care.

7.2. Virus Culture and Cell/Tissue Specimens

Methods for detecting viruses are either direct or indirect methods. Direct methods assay for the presence of the virus itself, while indirect methods observe the effects of the virus, such as cell death or the production of antibodies by the infected individual. Tissue culture is a way to identify a virus based upon the effects of the virus upon the cells. It is also a way to amplify virus if a larger sample is needed for other diagnostic tests, since viruses require cells to replicate.

Tissue culture, also known as cell culture when cells are grown specifically, involves maintaining living cells or tissues in a controlled environment outside a living organism. The cells are housed in plastic flasks or bottles and bathed in a liquid growth medium that contains nutrients and supplements ( Fig. 7.3A ). These cultures are grown in an incubator set to body temperature (37°C). A cell line is a set of cells that have been isolated from a tissue or organ fragment ( Fig. 7.3B and C ). Cell lines can be finite or continuous. Finite cell lines will only undergo mitosis a limited number of times, while continuous cell lines are immortal and will proliferate indefinitely. This characteristic is usually a result of genetic mutations. Cell lines derived from tumors, which have lost control of regulating the cell cycle and proliferate indefinitely, can also result in continuous cell lines. The choice of a finite versus continuous cell line for propagating viruses in culture will depend upon the virus that is needed to be isolated. Additionally, the cell line must be one that expresses the cell surface receptor specific for the virus and be permissive to infection; otherwise, the virus will not be able to attach and replicate within the cell line.

Cell culture.

(A) Three cell culture flasks containing living cells and culture medium (pink). (B) MDCK cells, the preferred cell line for isolating influenza A and B viruses. (C) Vero cells, which are susceptible to infection with herpes simplex viruses, poliovirus, Coxsackie B virus, respiratory syncytial virus, mumps virus, rubella virus, SARS-CoV, and lymphocytic choriomeningitis virus, among others.

Cell cultures must be grown using aseptic technique and sterile conditions, otherwise bacteria and fungi that get into the cell culture will grow profusely in the rich growth medium and contaminate the culture. A sterile environment is provided by a biological safety cabinet (BSC) , which is different from a fume hood that is used when working with chemicals. A BSC uses a fan to filter air through high-efficiency particulate air (HEPA) filters, which filter out bacteria, fungi, spores, and viruses to generate sterile air. Particles of 0.3 μM in size are the most penetrating through the filters but are still removed with 99.97% efficiency. There are three different classes of BSCs, designated class I, class II, and class III, that provide varying levels of protection to the worker and to the biological material being manipulated. Class I BSCs act much like chemical fume hoods, except that the air is filtered through a HEPA filter before it is released to the environment. The worker and environment are protected, but the biological material is exposed to nonsterile air from the environment so these are not used for cell culture ( Fig. 7.4A ). Class II BSCs are most often used, as they afford protection to the worker, the biological material, and the environment. The air entering the BSC from the front is sucked into a grille to prevent it from contaminating the working surface, which is constantly bathed in HEPA-filtered air to ensure a sterile working environment ( Fig. 7.4B and D ). Class II BSCs rely on the laminar flow of air, which means that the BSC creates an uninterrupted flow of air in a consistent, uninterrupted stream. As long as the work is performed within the stream of air, then the material will remain sterile. The air is also sent through a HEPA filter before being released to the environment. Class II BSCs are sufficient to work with cells and the majority of viruses. Some dangerous pathogens must be manipulated within a class III BSC, which is air-tight to prevent any exposure of the virus to the worker, who must use the heavy-duty rubber gloves that are built into the BSC to work with the pathogen ( Fig. 7.4C ). The air leaving a class III BSC is passed through two HEPA filters, or a single HEPA filter and incinerator, to ensure the pathogen does not enter the environment.

Classes of biological safety cabinets.

(A) Class I BSCs protect the environment by filtering contaminated air through a HEPA filter, but the cabinet does not provide a sterile environment within it. Class I BSCs are mainly used to house equipment that might generate aerosols but are not for use with sterile cultures. (B) Class II BSCs filter the environmental air through a HEPA filter before entering the cabinet to provide a sterile working surface. Environmental air is sucked into a grille at the opening of the cabinet to prevent it from contaminating the sterile working area. (C) Class III BSCs are completely sealed and air-tight, and all air entering or exiting the hood is filtered. Work in the cabinet must be performed by using the attached rubber gloves. (D) This researcher is performing cell culture in a class II BSC. Notice the grille at the front that prevents environmental air from entering the working area. The great majority of research and clinical laboratory work involving viruses is performed in a Class II BSC.

Appropriate levels of safety must be taken when working with viruses. Rhinoviruses cause colds, and so the effects of accidentally being exposed to rhinovirus are minimal and self-limiting. Other viruses, such as viruses that cause hemorrhagic fevers, can lead to deadly effects if someone is exposed. Therefore, there exist four biosafety levels (BSLs) that specify what precautions must be taken with different pathogens ( Table 7.3 ). Some notable differences between the BSL levels are summarized below:

• Biosafety level 1 (BSL1) is for work involving well-characterized agents not known to cause disease in healthy adult humans. These pathogens present minimal potential hazard to laboratory personnel and the environment. All material must be handled in an appropriate way and decontaminated after use, and workers must use gloves for protection, along with a lab coat and safety glasses, if warranted. Work with BSL1 agents does not require a BSC, unless cell cultures require the use of one to maintain sterility.
• Biosafety level 2 (BSL2) is for work with agents that are known to pose moderate hazards to personnel and/or the environment. It includes all the precautions of BSL1 but also requires that laboratory personnel are supervised, receive specific training in handling the pathogenic agents, and conduct any work that may generate infectious aerosols or splashes in a BSC.
• Biosafety level 3 (BSL3) is for work that could cause serious or potentially lethal disease through inhalation. BSL3 work includes all the precautions of BSL2 but also requires a special BSL3-level laboratory that is entered through two self-closing doors and is under negative pressure so that contaminated air is drawn to another area and HEPA-filtered before leaving the room. Laboratory personnel must wear additional protective laboratory clothing, such as a gown, scrub suit, or coveralls, that is only worn while in the laboratory and then decontaminated or disposed of upon exit of the lab. Work must be performed in class II or class III BSCs, depending upon the virus.

BSL4 protective suit.

This researcher at the CDC is using a light box to count viral plaques in a BSL4 laboratory. Note that the suit is inflated because of the air supply, which keeps it under positive pressure in case of a leak in the suit. The researcher also must wear laboratory clothing under the suit (scrubs in this case). Upon exiting the lab, the worker will pass through a bleach shower to decontaminate the orange plastic suit, change out of his laboratory clothing, and then take a personal shower before changing back into his street clothes.

Biosafety Levels Required for Work With Certain Viruses

The majority of diagnostic cell cultures take place at BSL2. Generally, three to six cell lines are selected based upon the test being performed and cell cultures are inoculated with the clinical specimen. Although viruses cannot be seen under the light microscope, they biochemically affect the cells in which they are replicating, which sometimes leads to visible cytopathic effects (CPEs) that are distinguishable using a light microscope ( Fig. 7.6A and B ). For example, some viruses may cause cells that normally attach to the bottom of their culture vessel to round up and detach. Large bubblelike vacuoles are sometimes observed in the cytoplasm of infected cells, which may also swell or shrink, depending upon the virus. Other viruses cause adjacent cell membranes to fuse together, creating a syncytium, or giant multinucleated cell, that can have up to 100 nuclei within the cell. Some viruses cause lysis of the cells in which they replicate. Other viruses cause cell death or damage by hijacking the cell’s transcription and translation machinery, leaving the cell at a deficit to translate its own proteins, including enzymes that are required for metabolic pathways.

Cytopathic effects of viral infection.

Viruses cause a variety of visible effects upon cells. (A) Human herpesvirus-6 infection causes ballooning of infected cells. (Image courtesy of Zaki Salahuddin and the National Cancer Institute.) (B) These photos show the cytopathic effects upon cells before (left) and after (right) infection with murine cytomegalovirus. Note the differences in morphology and organization of the cells. (C) A multinucleate syncytium caused by SARS-CoV in a histological section of stained human lung. (Image courtesy of the CDC/Dr. Sherif Zaki.) (D) A cytoplasmic inclusion body composed of viral proteins and nucleic acid is visible in this electron micrograph of a cell infected with Lagos bat virus. Note the bullet-shaped virions assembling from the inclusion body. (Image courtesy of Dr. Fred Murphy and Sylvia Whitfield at the CDC.)

Observation of CPEs—and how long it takes the virus to cause them—can provide clues for the diagnosis of the virus. For instance, adenoviruses cause cells to form grapelike clusters; herpesviruses cause cells to round up; and respiratory syncytial virus induces syncytia, as its name suggests. Other times, cell culture is used in conjunction with other assays. Electron microscopy can be used to identify the morphology of the virus, while immunofluorescence assays (IFAs) or immunohistochemistry (IHC), described in more detail below, can be used to definitely identify a specific virus within cells by recognizing viral antigens. For these assays, cells can be grown on slides or coverslips for easy removal from the culture.

Some types of clinical specimens bypass the need to infect cells in culture because they are infected cells or tissues taken directly from the patient. Cytology is the examination of cells, while histology refers to the examination of tissues. Blood, lung washings, and cerebral spinal fluid contain cells, and cells would also be collected during a cervical swab, which brushes some cells from the cervix and places them in a liquid preservative. The related Papanicolaou (pap) smear scrapes cells from the cervix but instead smears them across a slide that is then sent for analysis. A Tzanck smear uses a similar idea but is used to smear cells from skin lesions onto a slide for diagnosis of herpesvirus infections. These specimens would be subject to cytology. On the other hand, tissues are collected when a biopsy is taken. The tissue is sectioned into thin slices and undergoes histological examination at the diagnostic lab.

Most cells and tissue are devoid of color and therefore require colored stains for viewing under a microscope. Just as CPEs can be visualized in infected cell cultures, they can also be seen in cell or tissue specimens from a patient ( Fig. 7.6C ). Another type of CPE observed in infected cells or tissues is called an inclusion body, which is a visible site of viral replication or assembly within the nucleus or cytoplasm that can be observed with infection by some viruses ( Fig. 7.6D ; see also Fig. 13.9). Just as with infected cell cultures, cell or tissue specimens can be used for IFA or IHC.

7.3. Detection of Viral Antigens or Antiviral Antibodies

Although cell culture, cytology, and histology are valuable in providing visible clues as to the identity of a virus, some viruses cause similar CPEs or no visible CPEs at all. In these cases, it is necessary to use assays that detect viral antigens to prove the presence of a virus. This is accomplished by using antibodies that recognize specific viral antigens. Due to the advents of biotechnology, antibodies can be produced in large amounts and are commercially available. These antibodies recognize different antigens from a wide range of pathogens, including viruses. Several widely used and relatively fast assays make use of antibodies to identify the cause of a viral infection. “Immuno” is usually found in the name of the assay to indicate that it uses antibodies, which are produced by plasma cells of the immune system.

IFAs are performed on cells or tissues that have been affixed to slides and exposed to a fixative. The cells can be from patient specimens or they can be cell cultures that have been infected with patient samples. IFAs use antibodies that are conjugated to fluorophores, or fluorescent dyes, that give off a certain color when they are excited by a particular wavelength of light. The best-known fluorescent dye is fluorescein isothiocyanate , better known as FITC , which is excited by light of 490 nm (blue) and gives off light in the 519 nm range (green). In an IFA, FITC-conjugated antibodies specific for a certain virus are added in a liquid buffer onto the cells or tissue on the slide ( Fig. 7.7A ). If the viral antigen that is recognized by the antibody is present on the cell surface, perhaps because the virus was assembling at the plasma membrane or was in the process of infecting cells when the section was fixed, then the antibodies will bind to the viral antigen present. Alternatively, the cells can be permeabilized with a detergent to allow the antibodies to enter inside the plasma membrane and bind viral antigens there ( Fig. 7.7B ). In either case, the antibodies will not bind if the specific antigen is not present. After a sufficient period of time to allow binding of the FITC-labeled antibodies, the slides are washed with a buffer. Any antibody that is bound will remain bound, whereas unbound antibody will be rinsed off. The slides are examined under a fluorescence microscope, which contains a special lightbulb that can provide the wavelengths of light able to excite the fluorophores and filters that allow the viewer to see one emitted color at a time. In this case, if any green cells are seen, then the antibodies bound and the virus was present.

Immunofluorescence.

Immunofluorescence uses virus-specific antibodies to verify infection. (A) A buffer containing FITC-conjugated antibodies is added to fixed cells or tissues (from cell culture or cell/tissue specimens). The antibody binds to cognate viral antigens expressed on the surface of infected cells, which prevents it from being rinsed off the slide after incubation. The “stained” cells are examined under a fluorescence microscope to excite the FITC dye, which fluoresces green. This process can also be used to detect intracellular antigen by permeabilizing the plasma membrane to allow the antibodies to enter the cell (B). Some antibodies are not commercially available as already conjugated, so a FITC-labeled secondary antibody that recognizes the first antibody must be used (C). (D) is an example of an IFA performed on cells to verify the presence of respiratory syncytial virus. The green cells are therefore infected.

Because the antibody was directly bound to the specific antibody, this IFA is known as a direct IFA, or direct fluorescent antibody staining. However, sometimes conjugated antibodies are not available; in these cases, a second antibody must be used that is fluorescently conjugated and recognizes the primary antibody ( Fig. 7.7C ). This is a way to get around not having a conjugated primary antibody. It can also be used to amplify a weak signal when not much viral antigen is present. Because this assay requires a secondary antibody and the fluorochrome is not attached to the primary antibody, it is known as an indirect IFA. IFA is used for the identification of a variety of viruses, including several different herpesviruses, influenza, measles, mumps, and adenovirus ( Fig. 7.7D ).

IHC relies upon the same principles as an IFA except that the antibody is conjugated to an enzyme instead of a fluorescent molecule. After the tissue is exposed to the enzyme-bound primary antibodies, a liquid substrate is added to the slide. If the enzyme-linked antibodies have bound to the tissue, the enzyme will cleave the substrate and a visible colored precipitate will be deposited on the slide. IHC is visible using a normal light microscope and does not require the use of a fluorescence microscope. IFAs or IHC can take as little as a few hours to perform.

An extension of this concept takes place in an enzyme immunoassay (EIA) , also known as an enzyme-linked immunosorbent assay (ELISA) . Like IFA or IHC, EIAs/ELISAs also detect viral antigens using antigen-specific antibodies. Unlike IFAs or IHC, however, they do not use cells or tissues. Instead, they assay for viral antigens in liquid samples, such as serum or urine. A sandwich ELISA , starts by adding antibodies in a buffer to a special plastic plate that has been treated to bind proteins—including antibodies—to it ( Fig. 7.8 ). These are known as “capture antibodies” because they will be used to capture the antigen from the patient sample. Once the antibodies have bound, the wells of the plate—usually there are 96 of them—are rinsed with a buffer to remove any unbound antibody. The plate is blocked, meaning that a buffer is added that contains nonspecific proteins, which attach to the plastic wells wherever there is no antibody bound. The wells are rinsed again, and then the patient’s sample is added. If the antigen is present in the sample, either because the virus or pieces of the virus are present, then the antigen will bind to the capture antibody. In contrast, if no viral antigen is present in the patient sample, then the capture antibodies will not capture any antigen.

Enzyme-linked immunosorbent assay (ELISA).

An ELISA measures viral antigens (direct “sandwich” ELISA) or antiviral antibodies (indirect ELISA). In a sandwich ELISA (left), capture antibodies specific for the viral antigen are coated on the bottom of the wells, which are then blocked to prevent nonspecific binding. The wells are rinsed out in between each step to remove unbound molecules. The patient samples are added, one sample per well, at which point any cognate viral antigens will be bound by the capture antibodies. Next, enzyme-linked detection antibodies are added to each well and also bind the viral antigen, if present. When the substrate is added, it will be cleaved by the enzyme, if present, to form a visible color that is read by a spectrophotometer. In an indirect ELISA (right), the viral antigen is coated to the bottom of the wells and blocked. When the patient samples are added, any patient antibodies that recognize the viral antigen will bind to the plate, as well. Secondary enzyme-linked antibodies are added that recognize human antibodies, and the enzyme cleaves the substrate when added to produce color.

The wells are again rinsed to remove any leftover patient sample. Next, an antibody is added that is conjugated to an enzyme, just as with IHC. (This is where the “enzyme-linked” part of the ELISA name is derived; the “immunosorbent” part indicates that antibodies, the “immuno” part, absorb the antigen.) The enzyme-linked antibody, called the detection antibody, also specifically recognizes the antigen and will create a “sandwich” with the antibodies as the bread and the antigen as the meat—hence the name “sandwich ELISA.” After sufficient time for binding to occur, the wells are again rinsed. The detection antibody will remain bound to the plate if the antigen was present, but if no antigen has bound the capture antibody, then there will be nothing for the detection antibody to bind and it will be rinsed out of the well.

The final step is to add the liquid enzyme substrate. If the detection antibodies are present—meaning that the capture antibody bound antigen because the antigen was present in the patient sample—then the enzyme attached to the detection antibodies will cleave the substrate, producing a visible color in the well. If no antigen was present, then there will be no detection antibody to catalyze the substrate reaction. No color will occur.

Spectrophotometers measure the intensity of light, including colored light. A special 96-well spectrophotometer ( Fig. 7.9B ) measures the color in each well and provides an optical density value that indicates the relative amount of antigen present in each well. If a standard set of samples with known concentration is also tested in the same ELISA plate, then the value obtained with the patient sample can be compared to the standards to obtain a quantitative value. Positive and negative controls are always performed in each ELISA to verify that the assay was performed correctly.

(A) Note the variations in the color in the wells of the 96-well ELISA plate. A darker color indicates a higher amount of antigen or antibody was present in the patient sample. (B) An ELISA about to be read in a spectrophotometer.

Each step of an ELISA usually allows around an hour or two for binding to occur. Therefore, an ELISA takes several hours to perform all together, but results are usually available the same day the clinical specimen was received. Testing for the hepatitis B surface antigen is performed using a sandwich ELISA.

ELISAs can be used to not only detect viral antigens, but to determine the presence of antibodies against a virus, known as antiviral antibodies. Assaying for antiviral antibodies is an indirect way of seeing if a patient has been exposed to a virus, and ELISAs that differentiate between IgM and IgG can provide clues as to whether a patient has been newly exposed. If a person has IgM and becomes positive for IgG over the course of days, then the person is experiencing a primary infection. If the person has high levels of IgG, then it is a secondary or recurrent infection. Antibody isotypes are also used to diagnose congenital infections. IgG crosses the placenta from the mother to the child and is therefore not helpful in diagnosis, but IgM is too large to cross the placenta. Therefore, if IgM is present in the infant’s blood, it is made by the infant’s immune system and indicates an infection in the child.

To assay for antibodies, the viral antigen is coated onto the bottom of the ELISA plate ( Fig. 7.8 ). The patient sample is added, and if the patient possesses antibodies that recognize the viral antigen, then the antibodies will attach to the antigen on the plate, thereby immobilizing the antibody to the plate, as well. As with a sandwich ELISA, the next step is the addition of an enzyme-linked detection antibody. In this case, the detection antibody recognizes the patient’s antibodies. Finally, the wells are rinsed and a substrate is added. If the patient possesses antibodies against the viral antigen, the patient’s antibodies and subsequent detection antibodies would have bound to the antigen coated in the wells, and the presence of the enzyme would cleave the substrate, producing color ( Fig. 7.9 ).

A western blot utilizes a similar process, except that the protein antigens are immobilized in a gel first. In the same way that DNA can be separated using agarose gel electrophoresis, proteins can be separated by size using polyacrylamide gel electrophoresis (PAGE)( Fig. 7.10A ). The proteins embedded into the polyacrylamide gel are transferred onto a membrane made of nitrocellulose. At this point, the membrane functions like the wells of the ELISA plate. Enzyme-linked antibodies against the proteins that have been separated are added in a liquid buffer to the membrane and will bind specifically to the bands of protein antigens ( Fig. 7.10B ). A substrate is added, and the enzymes present produce detectable color, light, or fluorescence, depending upon the enzyme and substrate used ( Fig. 7.10C ).

Western blot.

Western blots work on the same principles as an ELISA, except the proteins are separated in a gel and transferred to a membrane instead of using wells. (A) To confirm a positive HIV result, a variety of different HIV antigens are separated using PAGE. The samples are loaded in the wells at the top of the gel, a charge is applied to the gel, and the proteins separate by size, with the smallest proteins traveling farthest in the gel. (B) Following PAGE, the HIV proteins are transferred to a nitrocellulose membrane. The patient serum sample is incubated with the membrane, and if the patient has anti-HIV antibodies, they will bind to the HIV proteins on the membrane. An enzyme-linked secondary antibody is added that recognizes human antibodies (just the enzyme part is shown here to save room). A substrate is then added that produces a detectable signal; in this case, the substrate gives off light that will create a dark mark when placed against X-ray film (C). In this scenario, the patient is positive for anti-HIV antibodies and thus has been infected with HIV.

Western blots are often used to confirm positive ELISA results. For example, if a person takes an oral HIV test (that uses saliva) and receives a positive result, the test must be confirmed using a western blot. In this scenario, HIV antigens are separated through PAGE, transferred to a nitrocellulose membrane, and exposed to the patient’s serum. If the patient has anti-HIV antibodies, they will bind onto the membrane where the HIV antigens are found. A secondary, enzyme-linked antibody will provide the enzyme that will cause a colored band to be produced when exposed to a substrate. (Western blots are still recommended for confirmation of oral HIV tests, although nucleic acid testing is now the choice of confirmatory test for positive HIV blood tests.)

Agglutination reactions take place when the binding of antibodies to antigen causes a visible clumping, or agglutination. Latex agglutination tests use the same principles as ELISAs or western blots, except the antigen or antibody is bound onto small latex beads ( Fig. 7.11A ). To test for patient antibodies, the viral antigen is coated onto the latex beads. When mixed with patient serum, the antibodies will bind to the antigen-coated latex beads. Because antibodies have two antigen-binding sites, each arm of the antibody is able to bind a different bead. The result is that the beads are splayed out in a lattice formation—the beads have agglutinated, and a visible “clump” has formed. If the patient does not have antibody against the particular viral antigen, then the latex beads will not agglutinate. Agglutination reactions can take place in tubes, 96-well plates, on slides, or using cardboard cards. Like ELISAs, latex agglutination tests can also test for viral antigen in a patient sample by coating antibodies onto the beads that recognize the antigen. Several antibody-coated beads will bind to one antigen, resulting in visible agglutination.

Agglutination/hemagglutination.

(A) Latex agglutination assays coat viral antigen onto latex beads. If antibodies are present that recognize the antigen, they will agglutinate the beads. The same procedure can be performed to assay for the presence of viral antigens by coating antibodies on the beads. (B) Hemagglutination refers to the agglutination of red blood cells specifically. Several viruses, including influenza virus and measles virus, possess hemagglutinin proteins that bind to red blood cell surface glycoproteins. Hemagglutination can be used to identify these viruses. (C) The results of a hemagglutination reaction that show hemagglutination occurring as increasing amounts of virus are titrated into the wells. The “button” on the left and the “clump” on the right show the visible change that occurs with hemagglutinated blood (on right).

Hemagglutination refers to the agglutination of red blood cells. A handful of viruses, including influenza virus, measles virus, mumps virus, rubella virus, and rabies virus, cause the hemagglutination of red blood cells. The viral attachment glycoproteins (aptly named “hemagglutinin” in influenza, measles, and mumps viruses) bind to the surface of red blood cells and agglutinate them, similarly to how antibodies do ( Fig. 7.11B and C ). Hemagglutination assays have been used to show the presence of these viruses in samples. A similar assay tests for the antibody levels in a patient sample against one of the viruses. A fixed amount of virus is added to each well, and then the patient serum is titrated into the wells at different dilutions. If the patient has antibodies against the virus, the antibodies will bind the virus and prevent it from hemagglutinating the red blood cells. By using dilutions of the patient serum, the relative amount of antibody can be determined by noting which dilutions do or do not prevent hemagglutination.

An assay that uses antibodies in a similar fashion as to the assays described above is called the lateral flow immunoassay (LFIA) . In this case, the presence of a virus (or antibodies against a virus, if the test assays for antiviral antibodies) will result in a colored band appearing in a particular window on the test. Pregnancy tests are the best-known LFIAs, but LFIAs are available that test for HIV antibodies, dengue virus antibodies, rotavirus, respiratory syncytial virus, and influenza virus. They can be in the form of a dipstick, like a pregnancy test, or they can be a small plastic test that requires a drop of sample be placed into a sample well ( Fig. 7.12A ).

Lateral flow immunoassay (LFIA).

(A) Lateral flow immunoassays have three major zones: a reaction zone, a test zone, and a control zone. The patient’s sample is added to the sample well of the assay and begins moving though the membrane via capillary action. In the reaction zone, antigens found in the patient’s sample bind to free antibody-coated beads. The bead-antigen complexes continue traveling to the test zone, where membrane-bound antibodies bind to the antigen, thereby immobilizing the beads. As the beads accumulate, a colored line is formed. Excess bead complexes continue moving through the membrane and are eventually bound by control zone antibodies that recognize the bead complex, rather than antigen. As excess beads accumulate here, a colored control line is formed. (B) LFIAs are useful for diagnosis of viral infection in animals, as well. This LFIA tests for two feline viruses, feline immunodeficiency virus and feline leukemia virus. Note that the control spot is present, but there are no other colored spots in the test zone area, indicating that the animal was negative for both viruses.

In the LFIA, the test reagents are added onto a nitrocellulose membrane that draws the liquid along the length of the strip using capillary action ( Fig. 7.12B ). The test strip contains three major zones: a reaction zone, test zone, and control zone. In the case of the influenza LFIA, the patient specimen (usually a nasal or nasopharyngeal swab) is mixed with a buffer that disrupts any virions that are present, releasing the viral antigens. A drop of the liquid sample is placed in the sample well on one end of the LFIA test, and the viral antigens begin to move through the membrane through capillary action to the reaction zone of the test. The reaction zone contains free antibodies that are bound to gold beads or blue latex beads, which will be the basis for the formation of color later in the test. As the viral antigens pass through the reaction zone, the bead-linked antibodies bind to the antigens, and the antigen–bead complex continues moving through the membrane. The test zone is the next zone encountered. In the test zone, other antibodies are immobilized to the membrane and recognize the viral antigens that are bound to the reaction zone bead–linked antibodies. The binding of the antigen–bead complex to the test zone antibodies immobilizes the antigen–bead complexes to the test zone area, creating an antigen sandwich. As the beads begin accumulating, the color of the beads becomes apparent. Blue latex beads provide a blue band, and gold beads appear as a red band. This indicates a positive result—that the person’s specimen contained the virus.

The final zone, the control zone, contains antibodies that will bind the bead complex. If the reaction zone beads are not bound to antigen, they will continue flowing through the test zone and be captured by the control zone antibodies. In fact, there are many more reaction zone beads than antigen, so some beads will be free even in the presence of antigen and make it to the control zone. The capture of the beads in the control zone produces a visible line in this section of the test. This is the positive control to show that the test worked.

The HIV or dengue virus LFIAs use the same principles, except they assay for the presence of antibodies against the viruses, rather than detecting the viruses themselves.

7.4. Detection of Viral Nucleic Acids

Nucleic acid testing (NAT) has replaced many of the traditional, slower assays in diagnostic labs. Detecting viral nucleic acids is a sensitive and specific way to screen for viruses within a patient sample, and new methods allow for the screening of many viruses simultaneously. Additionally, NAT is able to detect the presence of viruses for which no other test currently exists. NAT results are available the same day the sample is processed.

NAT assays rely upon the principle of polymerase chain reaction (PCR) , which recapitulates the process of DNA replication in the laboratory by providing the molecules necessary to copy DNA (see Chapter 3, Features of Host Cells: Cellular and Molecular Biology Review for a review). In the process of PCR, DNA (including any viral DNA present) is isolated from the clinical specimen, generally blood cells or tissue, and added to a tube containing primers, DNA polymerase, and nucleotides ( Fig. 7.14 ). The tube is placed in a thermocycler, a bench-top machine that simply changes the temperature of the tube, as its name suggests. In the denaturation stage of PCR, the thermocycler heats the DNA, usually to 95°C, which breaks the hydrogen bonds holding the two DNA strands together and so they separate from each other. In the annealing stage, the temperature is reduced to allow the binding (annealing) of two primers to the separated DNA. The primers are complementary to the sequence to which they bind, and they flank the region to be amplified, one primer on each strand. The annealing temperature is determined by the composition of the nucleotides in the primers, but is usually around 55°C.

Polymerase chain reaction (PCR).

PCR amplifies a specific sequence of DNA, based upon the location of primers. (A) In denaturation , high heat separates the two strands of the DNA sample. In annealing , the temperature is lowered, which allows the primers to hybridize to the strands. In extension , DNA polymerase extends the primers, creating the new strand. One DNA strand has now been copied into two. The three stages are repeated 30–35 times to generate billions of copies of the target sequence. If amplicons are created, then it means viral nucleic acid was present in the patient’s DNA sample. (B) A thermocycler, the machine that controls the reaction temperature during PCR. (The white machine on the right is another brand of thermocycler.)

In the final stage, extension , the thermocycler raises the temperature to the optimal temperature for the DNA polymerase enzyme. In this case, a special polymerase from the bacterium Thermus aquaticus , called Taq polymerase, is used. Thermus aquaticus was discovered to live in the hot springs of Yellowstone National Park, and its DNA polymerase is evolved to withstand high temperatures, such as those found in the hot springs. Taq polymerase is used for PCR because a human DNA polymerase would be denatured by the high temperatures required for the denaturation stage. The thermocycler holds the temperature at 72°C, and Taq polymerase extends

In-Depth Look: Counting Viral Particles Using a Plaque Assay

It is often necessary to know how many infectious virions are present in a sample. The diagnostic techniques described in this chapter identify the presence of a virus in a sample, or even the amount of viral nucleic acid, but these assays cannot determine the amount of virus present that is capable of productively infecting cells. A very common virology technique to determine this is known as the plaque assay , which measures the number of virions in a sample that are able to initiate infection of target cells.

Microbiologists measure viable bacteria by determining the number of individual bacterial cells in a sample, each of which will form a distinct bacterial colony. This results in the number of colony-forming units, or CFUs, in a sample. Plaque assays measure the number of individual cells that were infected by a single virion, each of which forms a plaque , or clearing, as the virus spreads among neighboring cells. This results in the number of plaque-forming units (PFUs) in a sample, the indication of viral infectivity.

To perform a plaque assay, the first step is to use cell culture to plate cells into several cell culture dishes or a multiwell plate—6-well or 24-well plates are often used for this purpose—in a liquid medium to support their growth ( Fig. 7.13 ). The cell line used must be permissive to infection with the virus that is being studied. The cells are allowed to grow to near confluency, meaning that they have grown to completely cover the bottom of the cell culture vessel. At this point, the medium is removed from the wells. Tenfold serial dilutions of the initial sample are made, in case the initial sample has too much infectious virus and ends up harming the entire well of cells, and 0.1 mL of the experimental samples are added to individual wells. The plate of cells is gently shaken on a flat surface, moving the plate in the motion of a “plus” sign, to ensure that the virus that was just added is equally distributed over all the cells.

A plaque assay determines the amount of infectious virus in a sample.

(A) Plaque assays are performed in a variety of different types of culture vessels, depending upon the size of the plaques that are formed. Shown here are a tissue culture dish, 6-well plate, and 24-well plate. (B) To perform a plaque assay, the undiluted “neat” sample is diluted several times using 10-fold dilutions. 0.1 mL of each dilution is added to an individual dish of cells. After allowing for time for the virus to enter the cells (about 1 h), a layer of agarose is used to overlay the cells to ensure lateral infection by infected cells. Depending on the virus, it generally takes 3–5 days for the visible sites of replication, called plaques, to be visible. At this point, the live cells are stained, and the clearings are counted (see Fig. 7.5 ). A dilution that has multiple but distinct plaques should be chosen to determine the PFU/mL in the original sample. In this case, the undiluted sample had 1.7 × 10 3 PFU/mL.

The virus is given time to bind and enter the cells (<1 h), at which point the cells are covered with a layer of cooled agarose that has been mixed with medium. Having entered random cells in the well, the virus will replicate over the course of the following days, and the agarose ensures that released virions are only able to infect cells immediately adjacent to the infected cell.

As the virus replicates and infects more cells, it begins forming plaques, or clearings, that are caused by the cytopathic effects upon the cluster of infected cells. In some cases, the virus is lysing infected cells, and in other cases, the virus has interfered with enough cellular processes to cause damage or death to the infected cells. Eukaryotic viruses typically take 3–5 days to form plaques that are large enough to see. Because cells are clear, a dye is used to stain the living cells, which leaves the clear plaques visible when the well is inspected.

The number of plaques are counted, giving the number of PFU per well. However, this needs to be converted into PFU/ mL in the original sample, so the number of plaques must be divided by the dilution of the sample and by 0.1 mL, since that was the amount added to the cells. Therefore, 0.1 mL of a 0.001 dilution (1/1000) that resulted in 54 plaques means that the starting sample had 540,000 PFU/mL in the initial sample—54/0.1 mL/0.001 = 540,000. It is best to write this using scientific notation: the amount of infectious virus in the starting sample was 5.4 × 10 5  PFU/mL.

the 3′-end of the primer using the available nucleotides, creating a double-stranded piece of DNA from the single-stranded template. In this way, one double-stranded piece of DNA was separated and replicated to create two copies.

The stages of PCR are repeated, usually 30–35 times, to create billions of copies of the target DNA segment. In the case of viral diagnosis, the primers would be specific for a piece of the viral genome. Good primers will bind to just the viral DNA being amplified and not to any isolated cellular DNA or genomes of other viruses.

What about viruses with an RNA genome? In this case, reverse transcriptase PCR (RT-PCR) is carried out. It involves the same steps as those performed with PCR, but because the viral genome is RNA, it must be reverse transcribed into cDNA first. This is accomplished using reverse transcriptase, which transcribes cDNA from an RNA genome. The reverse transcriptase used in RT-PCR is derived from retroviruses, generally Moloney murine leukemia virus or avian myeloblastosis virus. This additional step creates cDNA that is then amplified using PCR as above.

The amplified DNA fragment, known as an amplicon, can be detected in several different ways, but agarose gel electrophoresis remains the simplest and cheapest method. Agarose gel electrophoresis uses electricity to separate DNA fragments in an agarose gel. The distance traveled by the DNA is based upon the fragment size, with smaller fragments traveling farther in the gel than larger fragments. After separation is complete, the gel is stained with a chemical known as ethidium bromide that intercalates in between the base pairs of the DNA and fluoresces when exposed to UV light. A fluorescent band, therefore, indicates that DNA is present and was amplified by the PCR reaction. Because smaller fragments travel farther through the gel, the relative location of the band can verify the band that was produced is the anticipated size when compared to a known DNA ladder.

Multiplex PCR allows for the amplification of several different pieces of DNA at the same time in the same tube. This technique involves the same process as normal PCR or RT-PCR, but multiple primer pairs are added to the reaction tube. Each primer pair is designed to recognize the nucleic acid from a distinct virus, and the size of each amplicon produced is different for each virus. This allows for the simultaneous amplification of DNA from several viruses at one time, and analyzing the size of the amplicon reveals which viruses were present in the initial starting sample.

PCR was invented by Kary Mullis in 1983, and this molecular biology technique has been built upon and adapted to produce numerous assays of great utility. A modification of PCR that is ubiquitously used is known as real-time PCR , so named because the user can monitor the amplicon amplification in real time using a special thermocycler. A great advantage of monitoring the reaction as it proceeds is that the rate of amplification can be noted. Traditional PCR machines analyze the product after the completion of all cycles, which makes it impossible to know if the reaction had plateaued at an earlier cycle because of limitations in reagents, such as nucleotides. Real-time PCR machines address this concern by providing a visual graph of the amplification. An advantage of being able to see the amplification in real time is that the quantity of the PCR product can be used to back calculate the amount of starting template nucleic acid that was present, since we know that each cycle doubles the amount of DNA. For this reason, real-time PCR is also known as quantitative PCR. Therefore, the standard abbreviation for real-time PCR, is qPCR. (Recall that RT-PCR stands for reverse transcriptase PCR, not real-time PCR!) RT-PCR can also be performed using qPCR; this technique is known as RT-qPCR.

How is DNA amplification monitored in real time during qPCR? There are two major methods, and both involve the use of fluorescence. In the first method, a fluorescent dye is used that intercalates into double-stranded DNA, in the same way that ethidium bromide does. An example of a fluorescent DNA-binding dye is SYBR Green, which is excited with light of 488 nm and emits light at 522 nm (green). The real-time thermocycler is able to provide the excitation wavelength and has detectors to measure the emitted wavelength. Another method involves fluorescent reporter probes, such as Taq-Man ® probes ( Fig. 7.15A ). Like a primer, the probe recognizes a sequence of the DNA target, but it is located in the middle of the amplified sequence, in between the primers. Attached to the probe are a fluorescent reporter dye at one end and a quencher at the other. As long as the reporter dye and quencher are attached to the probe, the quencher absorbs the fluorescence emitted by the excited reporter dye (through the process of fluorescence resonance energy transfer, or FRET). Taq polymerase breaks down the nucleotides of the probe as it amplifies the DNA to which the probe is bound, releasing the reporter dye and quencher from the probe. When this occurs, the two are physically separated and the quencher molecule can no longer inhibit the reporter’s fluorescence. As with the fluorescent double-stranded DNA dyes, reporter dyes are excited and detected by the thermocycler.

Real-time PCR (qPCR).

Real-time PCR works on the same principles as PCR, except that it measures the fluorescence of a reporter dye in real time to monitor the amplification reaction. DNA-binding dyes are used, as are probes attached to a reporter dye and quencher molecule (A). The probe hybridizes to a section of the target DNA in between the two primers. As long as the reporter and quencher are physically close, the quencher absorbs any fluorescence and none is given off. As the primer is extended by DNA polymerase, the probe is broken down, releasing the reporter dye and quencher. Free from the quencher, the reporter dye fluoresces. The fluorescence increases as each cycle is repeated. (B) A real-time PCR amplification plot showing the amplification of eight different concentrations of JC polyomavirus (JCPyV) DNA. The number of reaction cycles is on the X axis, while the fluorescence generated in the reactions is on the Y axis. A few important things to note: The samples that start with more DNA are amplified more quickly (require fewer cycles to reach a set threshold, T). Also, each of the cycles eventually plateaus around the same fluorescence, even though we know some samples contain more DNA than others. This is why the samples are compared at the threshold limit (green line). This was multiplex qPCR: black lines are the amplification of a JCPyV protein–coding genomic sequence, while the red lines are the amplification of a noncoding genomic region.

Using either method, an increase in fluorescence indicates an increase in amplified product. A DNA amplification plot is created as the measurements of fluorescence are taken in real time ( Fig. 7.15B ). This allows the DNA product to be quantified at a cycle number where the amplification is in exponential phase, which can be used to back calculate the amount of starting template DNA. Multiplex qPCR assays have also been developed that allow for the simultaneous detection of several fluorescent probes in one tube, one for each amplified viral DNA segment.

PCR and qPCR amplify specific sequences of viral DNA. On the other hand, DNA microarrays rely upon the hybridization of viral nucleic acid segments to a synthesized piece of DNA. “Hybridization” refers to the complementary binding of two nucleic acid pieces to each other; for instance, primers hybridize with their target DNA during PCR. With DNA microarrays, thousands of short single-stranded pieces of DNA, called oligonucleotides (oligos), are immobilized onto a small silicon chip or glass slide ( Fig. 7.16B ). Oligos, like primers, are able to be synthesized in the laboratory using a specialized piece of equipment that bonds individual nucleotides together into a strand of nucleic acid. When detecting the presence of viral nucleic acid, the sequences of the synthesized oligo probes spotted on the chip are complementary to known sequences within the viral genome of interest.

DNA microarrays.

(A) Microarrays work on the principle of hybridization. For microarrays like the ViroChip that identify viruses within a sample, the starting DNA is copied using PCR. Fluorescent nucleotides are incorporated in the process. Then, the fluorescent amplicons are added to the microarray, which has spotted on it thousands of different oligonucleotide probes that each recognize a different viral sequence. If hybridization occurs with the sample DNA, then the virus’s nucleic acid was present in the starting sample. A machine measures the fluorescence at each spot. (B) An example of a microarray, found on the blue chip in the middle of the photo. Note the size of the chip, compared to the size of this researcher’s thumb. This chip contains 50,000 probes that hybridize to single nucleotide differences within the human genome. (C) Just a small portion (400 oligo spots) of an actual readout of a chip, showing in which spots fluorescence is present. This result used two different samples of cDNA, one labeled green and one labeled red.

Since viral nucleic acids may be in low abundance in a sample, microarray experiments begin with PCR to amplify the sample DNA ( Fig. 7.16A ). Primers are used that amplify randomly, rather than specifically, so that all DNA in the sample is amplified. Fluorescent nucleotides are incorporated into the amplified DNA. Following amplification, the fluorescent DNA is added to the microarray, and hybridization is allowed to occur between the sample DNA and the oligos on the microarray. Any hybridized DNA will remain bound to the oligos on the chip, while unhybridized DNA is rinsed away. The microarray is read by a machine that measures fluorescence. If fluorescence is present at a particular oligo spot, then the DNA from the sample is bound there and the machine will report a positive result ( Fig. 7.16C ). The identity of each oligo is known, and so the sequences present in the initial DNA sample can be determined.

Microarrays can be used for virus identification, but by modifying the oligos on the chip, microarrays can also identify strains of viruses that are particularly virulent or are genetically resistant to certain therapeutics. As long as the genome sequences are known that correspond to these attributes, then an oligo can be made that corresponds to the genetic trait.

Microarray principles can be employed for identification of RNA transcripts or proteins, as well. For example, instead of using DNA oligos to capture nucleic acids, protein microarrays use antibodies to capture different proteins.

An exciting application of the DNA microarray is the ViroChip, developed by the DeRisi and Ganem laboratories at the University of California, San Francisco. The ViroChip uses long, 70-nucleotide oligos (“70-mers”) as probes that are complementary to known viral sequences. The ViroChip contains over 60,000 probes that detect over 1000 viruses. The researchers included oligos for conserved and novel sequences found in related viruses. In this way, the ViroChip can identify similar viruses based upon conserved genomic regions and can also identify individual subtypes or strains based upon novel nucleotide sequences not possessed by related viruses. The ViroChip can also be used to characterize novel viruses. For example, it was used in 2003 to assist the CDC in identifying a novel virus that was causing deaths in Southeast Asia. The microarray suggested that the unknown virus was a previously unrecognized coronavirus, later to be known as the severe acute respiratory syndrome–associated coronavirus (SARS-CoV).

To create the oligo probes used on the ViroChip, researchers used the genome sequences of any viruses that had been sequenced. High-throughput sequencing methods allow for the determination of the nucleotide sequence of potentially any biological entity, including viruses. In the earlier days of viral diagnosis, CPEs were used to classify differences in subtypes or genotypes of viruses. This was replaced by serology and the use of antibodies to determine the viral subtype, and now sequencing of viral genomes provides a definitive differentiation at the nucleic acid level. Currently, over 4600 viral genomes have been completely sequenced, and public databases exist for the genomes of HIV, influenza viruses, dengue virus, and hepatitis C virus, among others. Sequencing also allows us to track the genetic differences between related subtypes and strains of viruses, and it assists scientists in identifying novel viruses by comparing genome sequences with those of known viruses. Bioinformatics is the field of study that uses computers to analyze and compare biological data, including genome and protein sequences. Bioinformatics is used to compare viral genome sequences, monitor viral evolution over time, and track virus mutations that appear during epidemics.

There are advantages and disadvantages to every method of diagnosis ( Table 7.4 ), which is why there are a range of tests available for the diagnosis and confirmation of commonly encountered viral infections ( Table 7.5 ).

Advantages and Disadvantages of Various Viral Diagnostic Techniques

Types of Diagnostic Tests Available for Well-Known Viral Infections

Summary of Key Concepts

Section 7.1 Collection and Transport of Clinical Specimens

• • Diagnostic tests are paramount in determining the etiology of viral infections.
• • Specimens can be collected from a variety of body fluids. The choice of specimen will depend upon the site and stage of infection and whether it is best to test for virus or antiviral antibodies.
• • Heparin or EDTA prevents the clotting of blood. Plasma is the liquid portion of nonclotted blood (and so contains clotting factors), while serum is the liquid portion of clotted blood.
• • Specimens must be carefully acquired, stored, and transported to ensure integrity of the samples. This is critical to ensure a meaningful and accurate test result.

Section 7.2 Virus Culture and Cell/Tissue Specimens

• • Direct diagnostic methods assay for the presence of the virus, while indirect methods test for effects of the virus.
• • Cell culture is the process of growing cells or tissues in the laboratory, using an incubator and special culture medium. Cells must be manipulated in at least a class II BSCs to maintain the sterility of the cultures.
• • Biological safety cabinets rely upon filtering air through HEPA filters, which filter out 99.97% of particles 0.3 μM in size. Class II BSCs are most often used in research and clinical diagnostic laboratories.
• • Certain viruses can cause severe effects and must be handled with caution. BSLs specify which precautions should be taken and are determined by the type of pathogen. They range in stringency from BSL1 to BSL4.
• • Cell lines can be infected with patient samples to allow viral replication within the cells. CPEs, such as morphological changes, ballooning, syncytia formation, or inclusion bodies, can help to identify the virus identity. Infected cells can also be used for IFAs.
• • Cytology and histology are the staining and microscopic examination of cell and tissue specimens, respectively.

Section 7.3 Detection of Viral Antigens or Antiviral Antibodies

• • There are a variety of immunoassays that use antibodies to identify viruses or antiviral antigens. The commercial availability of manufactured antibodies has revolutionized diagnostics.
• • Immunofluorescence assays are performed on fixed cells or tissue. Fluorescently labeled antibodies bind to viral antigens present in infected cells. A fluorescence microscope is used to excite the fluorophores so they give off colored light. IHC works in the same way except a colored precipitate is deposited at the site of the antibody.
• • ELISAs can be used to verify the presence of viral antigens or antiviral antibodies in liquid patient specimens. In a direct sandwich ELISA, capture antibodies that specifically recognize the viral antigen are coated on the bottom of an ELISA plate with 96 wells. If the patient sample contains the virus, then it will bind to the antibodies and become immobilized to the well. Detection antibodies are conjugated to an enzyme that will cause color change when a substrate is added. An indirect ELISA uses the same principles but coats viral antigen on the plate bottom to detect antiviral antibodies in a patient sample.
• • Western blots are sometimes used as a confirmatory test. They are analogous to indirect ELISAs, except the viral antigens are separated by PAGE and transferred to a nitrocellulose membrane before patient samples are added.
• • Agglutination reactions use antigen- or antibody-coated latex beads. If the patient sample contains the complementary antibody or antigen, the beads will agglutinate and form a lattice clump. Some viruses naturally hemagglutinate red blood cells.
• • LFIAs work like an ELISA in a stick. They rely upon a liquid patient sample traveling through a membrane and encountering antibody-coated beads that accumulate to cause a visible line.
• • Plaque assays measure the amount of infectious virus in a sample. It measures the number of plaques formed by allowing a single virion to infect a cell and laterally infect neighboring cells, forming clearings that become visible when the cells are stained. To determine the PFU/mL, the number of plaques is divided by the sample dilution and the volume added to the cells.

Section 7.4 Detection of Viral Nucleic Acids

• • Nucleic acid testing is a sensitive and specific way to identify viruses and viral subtypes/strains.
• • PCR recapitulates DNA replication in a test tube. Following the isolation of nucleic acid from the clinical specimen, a thermocycler uses heat to separate the two DNA strands. Primers anneal to a target sequence on each strand, and Taq polymerase extends the primer to create the complementary strand. The process is repeated 30–35 times to generate billions of copies of the amplified sequence.
• • Real-time PCR uses fluorescence to monitor PCR reactions in real time. It is quantitative because the rate of the reaction can be used to determine the initial starting material.
• • DNA microarrays rely upon the hybridization of fluorescently tagged DNA or cDNA to oligo probes coated on a glass slide or silicon chip. These are currently used for research purposes but have great potential for viral diagnosis.
• • High-throughput sequencing allows for the rapid determination of nucleotide sequences, including viral genotypes. It generates nucleic acid sequences that can be analyzed using bioinformatics.

Flash Card Vocabulary

Chapter Review Questions

• 1. You are a doctor. A patient shows up in your office that appears to have shingles, which is a reactivation of varicella zoster virus, the virus that causes chickenpox. How would you suggest going about definitively diagnosing her infection?
• 2. In the 2009 H1N1 influenza pandemic, specimens from patients with potential influenza infections were tested to verify the influenza subtype. What types of specimens might have been collected for such purposes?
• 3. Describe the observable CPEs that viruses induce in cells.
• 4. Explain how a class II BSC works to maintain a sterile working environment.
• 5. You are working with samples that contain human respiratory syncytial virus, which causes coldlike symptoms in adults. Which BSL is most likely required for work with this virus?
• 6. You have a serum sample, and you would like to verify whether it contains antibodies against a certain virus. Which of the following assays could be used for this? Explain why each is or is not appropriate to use: Immunofluorescence, ELISA, LFIA, cell culture.
• 7. List the steps involved in performing an indirect ELISA and direct sandwich ELISA. What is each used to measure?
• 8. You perform a plaque assay with different dilutions of virus, plating 0.1 mL per well of your cells. Your 1/10,000 dilution has no plaques, your 1/1000 dilution has 59 plaques, and your 1/100 dilution has too many plaques to count—they are not distinctive. How many pfu/mL are in your undiluted sample?
• 9. Describe what happens at each stage of PCR. How is real-time PCR performed differently?
• 10. You perform qPCR on two patient samples. Both show amplification of viral DNA. One sample crosses the threshold limit at 25 cycles, and the other patient’s sample crosses at 32 cycles. Which patient sample had more viral DNA in it?
• 11. Which type of test would be most effective in determining the entire nucleic acid sequence of a new strain of influenza virus?

Further Reading

• Centers for Disease Control and Prevention . December 7, 2012. Infectious Diseases Pathology Branch (IDPB) http://www.cdc.gov/ncezid/dhcpp/idpb (accessed 15.06.15.) [ Google Scholar ]
• Choo Q.L., Kuo G., Weiner A.J., Overby L.R., Bradley D.W., Houghton M. Isolation of a cDNA clone derived from a blood-borne non-A, non-B viral hepatitis genome. Science. 1989; 244 :359–362. [ PubMed ] [ Google Scholar ]
• Espy M.J., Uhl J.R., Sloan L.M. Real-time PCR in clinical microbiology: applications for routine laboratory testing. Clin. Microbiol. Rev. 2006; 19 :165–256. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Jerome K.R., Lennette E.H. fourth ed. Informa Healthcare USA, Inc.; New York, NY: 2010. Lennette’s Laboratory Diagnosis of Viral Infections. [ Google Scholar ]
• Kumar S., Henrickson K.J. Update on influenza diagnostics: lessons from the novel H1N1 influenza A pandemic. Clin. Microbiol. Rev. 2012; 25 :344–361. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Mahony J.B. Detection of respiratory viruses by molecular methods. Clin. Microbiol. Rev. 2008; 21 :716–747. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Mendelson E., Aboudy Y., Smetana Z., Tepperberg M., Grossman Z. Laboratory assessment and diagnosis of congenital viral infections: rubella, cytomegalovirus (CMV), varicella-zoster virus (VZV), herpes simplex virus (HSV), parvovirus B19 and human immunodeficiency virus (HIV) Reprod. Toxicol. 2006; 21 :350–382. [ PubMed ] [ Google Scholar ]
• Methods T.A., Chevaliez S., Rodriguez C., Pawlotsky J.-M. New virologic tools for management of chronic hepatitis B and C. Gastroenterology. 2012; 142 :1303–1313. e1. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Miller M.B., Tang Y.W. Basic concepts of microarrays and potential applications in clinical microbiology. Clin. Microbiol. Rev. 2009; 22 :611–633. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Quan P.L., Briese T., Palacios G., Ian Lipkin W. Rapid sequence-based diagnosis of viral infection. Antivir. Res. 2008; 79 :1–5. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Storch G.A., Wang D. Diagnostic virology. In: Knipe D.M., Howley P.M., editors. Fields Virology. sixth ed. Wolters Kluwer | Lippincott Williams and Wilkins; 2013. pp. 414–451. (Chapter 15) [ Google Scholar ]
• Tenorio-Abreu a, Eiros J.M., Rodríguez E. Influenza surveillance by molecular methods. Expert Rev. Antiinfect. Ther. 2010; 8 :517–527. [ Google Scholar ]
• United States Centers for Disease Control and Prevention and Association of Public Health Laboratories . June 27, 2014. Laboratory Testing for the Diagnosis of HIV Infection: Updated Recommendations. http://stacks.cdc.gov/view/cdc/32447 (accessed 15.06.15.) [ Google Scholar ]
• U.S. Department of Health and Human Services . Biosafety in Microbiological and Biomedical Laboratories. U.S. Department of Health and Human Services; 2009. Section IV—Laboratory biosafety level criteria; pp. 30–59. [ Google Scholar ]
• Wang D., Coscoy L., Zylberberg M. Microarray-based detection and genotyping of viral pathogens. Proc. Natl. Acad. Sci. U.S.A. 2002; 99 :15687–15692. [ PMC free article ] [ PubMed ] [ Google Scholar ]
• Wong R., Tse H. Springer; New York, NY: 2009. Lateral Flow Immunoassay. [ Google Scholar ]

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How to avoid computer viruses

In 2017, the “WannaCry” virus had the world’s biggest companies heading for disaster and home internet users trembling. It attacked a Windows blind spot to disable devices and demanded huge sums to unlock them. News soon spread across the planet and raised questions about internet security.

Though computer viruses have been around for decades, false alarms about them on the Internet are common. While the risks are always there, a lot of work goes into dispelling fake news .

The first known virus, named “Creeper”, occurred in 1972. Though engineer Bob Thomas created it without malicious intent, it attacked the Tenex operating system used on computers in the US for teaching and research. It made its way around the Internet carrying the message: “I’m the Creeper, catch me if you can”.

“Reaper”, considered the first ever anti-virus software , was created to fight it. While the first malicious viruses emerged in the 1980s, hundreds have surfaced since. Some have been extremely aggressive. Many experts point to “Chernobyl” as the worst ever, though it remains a strong topic of debate. Created in 1998, it attacked 60 computers operating on Windows and Windows ME. It erased all data and shut devices down.

Types of virus

One of the first malware viruses was “Rabbit” , which locked devices. Then came “Elk Cloner”, which affected Apple devices; and “Brain”, which hit IBM computers . This led to a list of virus types being drawn up:

• Resident. Found in a computer system’s memory, they infect files as they’re being selected or closed.
• Direct action. Installed in the root directory of the hard drive, they attack when you run an infected file.
• Overwrite . Found in files, they erase and replace content.
• Directory. Also known as “cluster” viruses, they change the location of files when we try to open them.
• Boot sector. They attack when you start your computer.Other types of secondary viruses

Secondary viruses

• Trojan horses. They hide behind harmless-looking programs and allow hackers to take control of your device to steal personal data . The most famous was “Zeus”, which affected over three million devices in the US, 77% of which had up-to-date anti-virus software.
• Worms. Malware that spreads when we transfer files. They block systems and waste bandwidth. The most talked about was “Morris”, which affected tens of thousands of Internet-connected computers.
• Spyware. They gather data from your device and transfer it to another location. One of the most threatening is “Skygofree”, which affects smartphones and allows hackers to take full control of the device.
• Adware. They track your computer’s memory to find out your preferences and bombard you with advertisements.
• Browser hijackers. They take control of your browser and direct you to other websites.
• Email virus. They appear when you open an email. The most famous was “I LOVE YOU” from 2000, which took just five hours to infect 10% of the computers connected to the Internet and caused EUR 10 billion worth of damage.
• Ransomware. They block your device and demand a ransom to unblock it.

How can we protect ourselves?

While the best way is antivirus software, operating systems already come with programs like Windows Defender and Windows Security. There are also other, free programs like Avast and Kaspersky.

They may seem simple, but they’re highly effective at destroying Trojan horses, worms and spyware. It’s also important to keep on top of updates. There are other ways to prevent viruses infecting your devices:

• Take special care on social media. Never open files without checking their source.
• Close websites when the browser tells you they’re not secure.
• Do not accept files from people you don’t know.
• Back up your files regularly.

1. Install antivirus or anti-malware software

It might seem obvious, but many home computers don’t have this protection. It's essential to keep your PC virus free.

2. Keep your antivirus software up to date

Protective software is one thing; but keeping it up to date is another. While free antivirus software is better than nothing, it’s not the best solution. Microsoft has a free security package if you operate with Windows, even though you would’ve already paid for the Windows licence. Many people don’t know about it; but, actually, it’s a good form of protection.

3. Run antivirus scans regularly

This might also go without saying, but we often forget to do it. Adjust the settings so scans run at regular intervals (like once a week). Using the device while antivirus software is running can be challenging. Try running it at night when the computer is idle. Because we usually turn our devices off at night, we tend to overlook scans. Set the antivirus software to run on a specific night and only leave the computer on at that time. Make sure it doesn’t switch off automatically or go into hibernation mode.

4. Keep your operating system up to date

Whether you use Windows, Mac OS X, Linux or another operating system, always keep it up to date. Developers regularly release patches to plug security leaks. The patches will help keep your system safe. You should also keep your antivirus software up to date. New viruses and malware are emerging constantly. Their software scanning is as sophisticated as their databases, so make sure you’re on top of things.

5. Protect your network

Many PCs connect to files, printers and the Internet via Wi-Fi .  Make sure the network requires a secure password and never browse on open networks.

Use WPA or WPA2 encryption. PME is no longer secure enough. Expert hackers can circumvent it in minutes. It's also a good idea not to disclose the name of your Wi-Fi network (the SSID). You can connect to the network manually on your device by typing in the SSID and password. If you usually let guests use your Internet, give them an alternative SSID and password just in case. 

6. Think before you click

Avoid websites you don’t trust. Don’t open email attachments from people or companies you don’t know. Don’t click on links in unwanted emails. Always hover the mouse over a link (especially a short URL) before clicking on it to see where it will take you.

If you need to download something from the Internet, an email, an FTP site, a file exchange service, etc., check it over first. Good antivirus software will do it automatically, although you have to make sure it's running.

7. Keep your personal information secure

This is probably the hardest thing to do on the Internet. Many hackers use social engineering over brute force to access your files. They can gather enough information to hack your online accounts to collect even more data.

They go from account to account until they have all they need to get hold of your bank details and steal your identity. Be careful on message boards and social media. Block all your privacy settings and avoid using your real name in chat forums.

8. Don’t use unsecured Wi-Fi

Don’t use the free, open Wi-Fi (no password or encryption) in cafés, libraries, airports, etc. Think about it. If you can connect easily, how far can a hacker go?

9. Back up your files

Backing up all your files is the best form of protection. Ideally, keep your files in three places: where you work on them (your computer); an external storage device; and somewhere else.

Use a back-up service or get two external hard drives and keep one at work; a relative or a friend’s house; or in a safe.

10. Use several secure passwords

Never use the same password twice , especially for bank accounts. We usually use the same email address or username, which are easy to see and steal. If you always use the same password and someone uncovers it, it’ll take just a few seconds to hack into all your accounts.  Choose a strong password with lower- and upper-case letters, numbers and symbols. Make it easy to remember but difficult to predict. Don’t use dates or pets’ names.

Computer virus myths

• They infect through email. It's one way they can, but it’s not the only way. They can also enter via the Internet, social media, etc.
• My computer is slow... it must have a virus! It’s possible; but your sluggish device could be due to poor maintenance, hard drive faults or other reasons.
• If the screen goes blue, it’s a virus. Blue screens usually mean faulty software, not a virus.
• Pornographic sites have viruses. This is becoming less and less true. The pornography industry is big business and boasts increasingly tighter online security.

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Types of computer viruses.

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A computer virus is defined as a malicious program or piece of code that self-replicates and in the process spreads itself to other executable files (Torres, 2017). Viruses are capable of corrupting system files, destroying data and wasting essential computer resources like Random Access Memory and storage space (Torres, 2017). Viruses can be spread through removable media, networks, emails, and downloads from the internet. Viruses can be classified based on how they infect the computer. Types of viruses include boot-sector viruses, file viruses, macro viruses, script viruses, email viruses, direct action viruses, memory resident viruses, non-resident viruses, polymorphic viruses, multipartite viruses, stealth viruses, sparse infector viruses, companion viruses, cavity viruses, armored viruses, and overwriting viruses.

Boot-sector viruses primarily infect the master-boot record ultimately loading concurrently with the operating system during start-up (Torres, 2017). They interfere with the booting process, data retrieval and can even delete partitions making computers unstable. Boot-sector viruses spread through physical media. An example of a boot-sector virus is the stoned-marijuana virus. Alternatively, file viruses or file-infecting viruses target executable files with the aim of permanently destroying them or rendering them unusable. A file-infecting virus replaces existing code with infectious code in an executable file (Torres, 2017).

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Macro viruses infect macros usually associated with data files such as Word documents and Excel spreadsheets. (Sebastian, 2013). A macro is a set of commands used to automate tasks within an application a software program. Macro viruses can imitate harmless macros to perform a sequence of operations without the knowledge of the computer user. A classical example of a macro virus is the Melissa which opened Microsoft outlook, accessed the user’s email address book and subsequently sent email copies of itself to the first fifty contacts found (Torres, 2017). The difference between script viruses and macro viruses isn’t very clear. However, script viruses are commonly found in web pages and are executed when a user visits infected websites or opens infected email file attachments.

Email viruses are spread by opening a file attached to an e-mail or by opening an email whose body has been infected. An infamous email virus is the “I love you” virus. Direct Action Viruses embed into specific files commonly EXE or COM files and get propagated. After executing their functions, they self-delete. Direct action viruses are the most common type of viruses around. They are effortlessly created and the easiest to remove from computers. A well-known direct action virus is the Vienna virus which looks up for .com files and destroys vulnerable ones in the process of infecting them (Torres, 2017).

Memory resident viruses stay in the computer’s random-access memory making them quite dangerous as they are difficult to detect and continue to work even if its source has been neutralized. A notable memory-resident virus is the Jerusalem virus/ Friday 13th virus which concealed itself in the computer’s RAM and proceeded to delete programs on the Friday 13th while inflating the sizes of infected programs till they were impossible to run (Torres, 2017). Another type of virus is a non-resident virus which actively searches for files to infect either on removable, network or local locations after which they remove themselves from the memory (Sebastian, 2013). They don’t reactivate until the next infected host file is executed.

Polymorphic viruses refer to types of viruses that frequently mutate to avoid detection which maintaining its potential to cause harm. They attack new files using altered and encrypted copies of themselves. Polymorphic viruses vary code sequences and create different encryption keys rendering identification by antiviruses difficult (Husain & Suru, 2014). An example is the Satanbug virus which gave antivirus software a very difficult task with its nine levels of encryption (Torres, 2017). Stealth viruses disguise themselves from virus scanners by masking the size of the files they are hiding in or temporarily removing themselves from the infected files. They then copy themselves to another location and replace the infected file with an uninfected one. A prominent example is the Frodo virus.

Multipartite viruses are versatile by combining the powers of boot-sector viruses and file-infecting viruses (Torres, 2017). Ridding files of this virus does not in any way guarantee that the boot-sector is safe and vice versa. An example is the tequila virus which added itself to the hard disk, altered partition data and modified the Master Boot Record to redirect to it. Sparse infector viruses infect only occasionally after certain conditions are met. This enhances their ability to avoid detection. An illustration of this is a virus which becomes infective only after a file is executed for the 20th time (Texas State University, 2017).

Armored viruses are designed to shield themselves from analysis by making disassembly, tracing and reverse engineering of its code cumbersome (Texas State University, 2017). Companion viruses exploit a property of DOS that allows executable files with the same name bearing different extensions such as .com or .exe to be run based on different priorities (Texas State University, 2017). This type of virus may generate a .com file that is given more priority than an .exe file sharing the same name. In contrast, cavity viruses overwrite a section of host program files specifically targeting the empty spaces. This in effect does not increase the length of the file making the program functional while the virus shields itself from detection (SebastianZ, 2013). Overwriting viruses destroy their host files by copying their code over them. Despite antiviruses being capable of disinfection, recovery of the affected files is usually impossible (Texas State University, 2017).

• Husain. R & Suru.S (2014). An Advance Study on Computer Viruses as Computer architecture. Retrieved from http://www.academia.edu
• Sebastian, Z. (2013). Security 1:1 – Part 1 – Viruses and Worms | Symantec Connect Community. Retrieved from https://www.symantec.com/connect/articles/security-11-part-1-viruses-and-worms
• Texas State University (2017). Virus Types: Information Security Office: Texas State University. Retrieved from http://infosecurity.txstate.edu
• Torres, G. (2017). What Is a Computer Virus? | The Ultimate Guide to PC Viruses. Retrieved from https://www.avg.com/en/signal/what-is-a-computer-virus

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Home » Projects » Direct Acting Antivirals for Pandemic Prevention

Direct Acting Antivirals for Pandemic Prevention

Development stage.

Health Condition & Disease

In the past, antiviral drug discovery focused almost exclusively on viruses that cause chronic, life-long, diseases, like AIDS, herpes and hepatitis.  The COVID-19 pandemic has revealed that this was a tragic mistake. Potent antivirals, like those discovered many years ago for the related SARS and MERS coronaviruses, were never developed for clinical use.  Millions of lives could have been saved if these compounds had been developed into drugs.

Direct-acting antivirals (DAAs) are defined as those that specifically inhibit viral enzymes that are needed for virus replication. Examples include the herpes drug acyclovir , AZT , the first drug approved to treat AIDS, and the hepatitis C drug sofosbuvir .  The term was coined to differentiate DAAs from other antiviral drugs, like interferon and ribavirin , which function mainly by modulating the host immune system.

For over 20 years , the Frick lab worked with numerous other labs and companies to develop DAAs to treat hepatitis C.  Many of these drugs are now used routinely in the clinic to cure chronic hepatitis C, which used to kill tens of thousands of Americans each year. With DAAs hepatitis C can now be cured with a single daily pill taken for eight weeks, with few if any side effects.  The goal now is to adapt these DAAs for use with other viruses, like SARS-CoV-2 , the virus that causes COVID-19.

Drug Target Identification

Most hepatitis C virus DAAs were discovered by biochemists using recombinant purified proteins because, unlike other viruses, HCV cannot be easily grown in the laboratory.  To facilitate these efforts, scientists in the Frick lab, isolated viral proteins and designed assays suitable for high-throughput screening. These enzymes include viral polymerases , proteases , primases , and helicases .  One example is an assay that can be used to discover viral helicase inhibitors and simultaneously differentiate them from toxic compounds that simply block viruses by binding nucleic acids (Fig. 1).

Figure 1.  The Molecular Beacon-based Helicase Assay (MBHA).   Helicases are molecular motors that re-arrange nucleic acids in reactions fueled by ATP.  By using DNA that forms hairpins, an MBHA can be used to simultaneously differentiate compounds that inhibit helicases from compounds that might block helicases simply by binding the double helix. DNA binding agents are typically toxic and less promising drug candidates.  For details see Frick Lab papers in Biotechniques and Methods in Enzymology .

Because we desperately need DAAs to treat COVID-19, beginning in January 2020, the Frick Lab began isolating proteins from SARS-CoV-2.  The 29,900 nucleotide SARS-CoV-2 RNA genome encodes many potential DAA targets, most of which are encoded by the rep1ab open-reading frame.  The Frick lab has had most success with the multifunctional 945 amino-acid-long nsp3, which is tethered to the ER with two ubiquitin-like domains, two papain-like protease domains, three macrodomains (Mac1, Mac2, and Mac3), a nucleic-acid-binding domain, and a hypervariable region (Fig. 2).

Figure 2. Attractive SARS-CoV-2 DAA targets.   The above plot shows amino acids conserved (green) between SARS-CoV-2 and related viruses in the 1ab reading frame.  The 16 mature nonstructural proteins (nsp) are shown with arrows with functions noted below. Nsp3 is highlighted. The figure is adapted from Fields Virology (6 th edition).

In Vitro Efficacy

After DAAs were approved and became the standard of care to treat hepatitis C, the Frick Lab began testing DAAs against similar viruses with RNA genomes like Dengue virus, West Nile virus, Zika virus and most recently SARS-CoV-2.  Many were potent enzyme inhibitors and blocked virus replication (Figs. 3, 4).  We now seek to develop this leads into antiviral drugs.

Figure 3. DAAs targeting the helicase encoded by Dengue virus.  The structure on the left is the Dengue virus NS2B/NS3 complex showing the binding sites for RNA, ATP and the various classes of DAAs discovered in screening campaigns.  Tables on the right show chemical structures and potencies in vitro and in cell based (replicon) assays and the PubChem compound identification (CID) number.  For more information see our paper in ACS Infectious Diseases .

Figure 4. Identification of FDA approved Drugs that might be templates to design DAAs for SARS-CoV-2.   (A) Design of an HTS-compatible ADP-ribose binding assays to detect inhibitors of the Mac1 domain of SARS-CoV-2 nsp3. (B) Virtual screens using AutoDock Vina to predict where compounds might bind Mac1.  (C) When libraries are screened with these assays desirable drug candidates should bind with lowest free energy and highest T m .  (D) Two similar FDA-approved proton pump inhibitors that bind Mac1.  Note that others have shown that omeprazole is a modest antiviral in cell culture .  Clinical trials with these drugs have been initiated .  For more information see our paper in SLAS Discovery .