A computer virus is a malicious program that invades vulnerable devices to alter the way they function. There are many types of viruses, and new ones are constantly being created. This means that you need to have a robust security plan in place to deal with these ever-changing threats.

Replication is the singular feature that makes viruses so dangerous to computers. Once inside the host program, the virus will start to make copies of itself, spread rapidly, and cause widespread damage to other software programs on your device. It can even spread across networks and reach any system connected to the network, damaging other endpoint devices and taking total control of the entire system. The virus can be challenging to eliminate once it gains access to the system.

In this article, we will discuss how viruses work, their common types, and warning signs. We will also guide you through steps to remove them from your device and offer some practical tips to avoid viruses moving forward.

What is a computer virus?

A computer virus is an illegal, harmful, or malicious program that can change the way your computer runs and behaves. Some of these programs seek to cause as much damage as possible to the endpoint devices they infect, such as altering software programs or corrupting data. Other viruses will attempt to gain access to sensitive information. This information might include personal identifying information, credit card numbers, bank information, addresses, and other financial data. Other viruses will seek to do both: steal your information and cause damage to your system.

The infection lifecycle

A computer virus follows a predictable lifecycle that security experts have studied extensively, and knowing these steps empowers you to recognize and prevent infections.

Computer viruses are typically delivered to your device through methods that rely on your own actions. Many viruses depend on you clicking links, opening attachments, or downloading software from untrusted sources.

  • Opening phishing email attachments that appear legitimate but contain malicious code
  • Attaching unsecured removable or external storage devices such as flash drives, external hard drives, and memory cards to your own device
  • Visiting compromised websites that trigger drive-by downloads
  • Clicking malicious ads
  • Downloading malicious and unlicensed applications

According to the FBI’s Internet Crime Complaint Center, these social engineering tactics remain the primary infection vectors. However, exploit-based viruses can infect systems automatically by taking advantage of unpatched software vulnerabilities, requiring no action from you.

Once embedded into your device or network, the virus starts the execution stage, and this is where your actions matter most. Many viruses require you to open an infected file, click a malicious link, or grant permissions before they can activate. However, some sophisticated viruses exploit software vulnerabilities to execute without your direct involvement.

During replication, the virus creates copies of itself and attacks different parts of your system. It plants its code into programs, system files, or documents, spreading each time you use the infected applications. This self-replication distinguishes viruses from other types of malware.

The persistence stage ensures the virus survives system restarts and removal attempts. Advanced viruses will modify system registries, create hidden files, or integrate themselves into critical system processes through multiple mechanisms to maintain their presence.

Finally, the virus executes its payload—the actual damage it was designed to inflict. This could range from data theft and system corruption to providing unauthorized access for cybercriminals.

The damage that computer viruses bring

Computer viruses can quietly disrupt your system long before you notice anything’s wrong. Once they infect your device, they can corrupt files, steal personal information, slow down performance, or even take full control of your system. Some viruses simply cause inconvenience, while others lead to severe data breaches and financial loss. Here’s a deeper look at some of the nefarious ways in which viruses can harm you and your device:

  • File corruption and deletion: Computer viruses can corrupt or completely delete your important files, photos, and documents. You might notice files won’t open, appear damaged, or suddenly disappear from your computer.
  • Data and credential theft: Viruses can steal your personal information, including passwords, credit card numbers, and banking details. Warning signs include unfamiliar account activity, unexpected password resets, or new accounts opened in your name.
  • System performance issues: Infected computers often run slower, freeze frequently, or crash unexpectedly. You may notice programs taking longer to load, unusual pop-ups appearing, or your computer becoming unresponsive during normal use.
  • Unauthorized communications: Viruses can hijack your email and social media accounts and send spam or malicious messages to your contacts. Friends might tell you they’ve received strange messages from your accounts, or you may see sent emails you didn’t create.
  • Network vulnerabilities: Through your home network, an infected device can spread threats to other connected devices such as smartphones, tablets, or smart home systems. Multiple devices may begin showing symptoms simultaneously, or you might notice unusual network activity.
  • Financial and identity risks: Computer viruses can lead to identity theft, unauthorized financial transactions, and long-term credit damage. The FBI’s Internet Crime Complaint Center reports billions in losses annually from cybercrime affecting personal computers.

Beyond the financial losses suffered by thousands of victims, the magnitude of a virus attack also includes cost of repair and recovery, as well as the opportunity loss to your professional and business relationships. These consequences, combined with the mental and emotional repercussions on a personal level, are excellent reasons to stay away from any suspicious sites, links, attachments, videos, images, or files that could be infected with a computer virus.

Viruses can corrupt trusted channels

While viruses can get on the system when the computer user clicks on an unsafe link, other viruses could enter the system through your normally trusted methods and might even sneak in without being detected if you install software that would otherwise be safe, infecting the deepest levels of your computer. By the time the damage is apparent, it could be too late. Some viruses target the user’s email account even if the malicious link or attachment appears to be coming from a trusted source.

This is why preventing infection the best course of action. Monitor your behavior online, and avoid any potentially infected sites or links. To detect any malicious programs and keep viruses at bay, you can employ antivirus software that will scan the system.

Common types of computer viruses

Viruses, worms, and Trojans are often confused with one another, but in fact behave differently. Viruses require a host program to replicate and spread, attaching themselves to existing files. Worms, in contrast, are self-contained and can spread across networks without user intervention. Trojans masquerade as legitimate software but don’t replicate—they simply provide backdoor access to your system.

Computer viruses can compromise your system and all of the information on your hard drive. Knowing about the types of computer viruses and their specific behaviors can aid you in identifying the best course of action to take.

Web scripting virus

This harmful virus hides in the coding of links, videos, advertisements, or code within a malicious website. If the user accesses an infected malicious website, the virus will enter the system and cause harm by breaching the web browser security.

File infector .exe

This type of virus attacks the computer after an infected file is launched. Most file infector viruses only replicate and spread, taking up storage, but others damage host programs and licensed applications.

Boot sector virus

A boot sector virus targets the boot sector of your storage devices—the critical area containing code needed to start your operating system. It typically spreads through removable media such as USB drives, external hard drives, or memory cards, and embeds itself in the Master Boot Record or Volume Boot Record, allowing it to activate before your operating system loads.

Among the noticeable symptoms of this virus infection are slow boot times, system crashes and unusual error messages during startup, or complete inability to boot properly. Fortunately, modern computer security has significantly reduced boot sector virus risks by verifying that boot software is digitally signed and trusted before it is run, making these infections much more difficult for cybercriminals to execute.

Network virus

Network viruses exploit shared resources and vulnerabilities between connected devices. Unlike traditional viruses that require direct file transfer or downloads, network viruses can automatically move from one device to another through your home Wi-Fi network, transferring from an infected laptop to your smart TV, gaming console, or any device connected to your network, potentially compromising your entire digital household.

Polymorphic virus

Polymorphic viruses are digital shape-shifters that can change their code signatures each time they replicate using encryption and code obfuscation techniques to create countless variations of themselves, making them challenging for traditional antivirus software to detect.

To counter this type of virus, today’s advanced antivirus systems use behavioral analysis and heuristic detection methods that focus a program’s activities rather than its looks to monitor unauthorized file modifications, unusual network communications, or attempts to disable security features. Cloud-based threat intelligence also plays a crucial role, allowing security providers to share information about new threats in real-time across their global networks.

Multipartite virus

A multipartite virus spreads through multiple infection methods to maximize its persistence on your system. Unlike single-method viruses, it can simultaneously target both your computer’s boot sector and executable program files.

This dual-attack strategy allows the virus to maintain its presence even when one infection vector is eliminated. If you remove the virus from your program files, it can still reinstall itself from the infected boot sector when you restart your computer and vice versa, continuing to cause damage.

Earning signs of a multipartite virus infection include frequent system crashes, programs launching more slowly than usual, error messages when opening applications, and your antivirus software repeatedly detecting and removing the same threats. You may also experience unusual hard drive activity during boot-up or notice that your system takes significantly longer to start than normal.

Real-world computer virus examples

  • Melissa Virus (1999): The most dangerous computer virus in 1999, Melissa mainly affected Outlook emails, spreading by sending copies of itself in a Word document to the first 50 addresses in a user’s address book.
  • ILOVEYOU virus (2000): Spread through email attachments with the subject line “ILOVEYOU,” the virus replicated itself to overwrite files and steal passwords from millions of computers worldwide.

Signs of a virus in your computer

Early detection is one of the vital defenses against a computer virus—after active prevention. Here are some of the indications that your computer might have a virus:

  • Sudden computer slowdowns: Your computer takes much longer to start up, open programs, or complete basic tasks that used to run smoothly.
  • Unexpected pop-ups and ads: You see frequent advertisements, warning messages, or browser pop-ups even when you’re not browsing the web.
  • Browser redirects: Your web browser automatically takes you to unfamiliar websites or your homepage has changed without your permission.
  • Unknown programs: You notice new software, toolbars, or desktop icons that you didn’t install, or programs launching automatically at startup.
  • Disabled security tools: Your antivirus software stops working, won’t update, or you can’t access security settings that were previously available.
  • Storage spikes: Your hard drive space decreases rapidly without explanation, or you notice unusually high disk activity when your computer should be idle.
  • Unusual network activity: Your internet connection becomes slower than normal, or you notice unexpected data usage even when you’re not actively online.

Remove a computer virus

Discovering a virus in your computer can be stressful, but with the right steps and reliable antivirus tools, you can safely eliminate malicious programs and get your device back to normal.

  1. Disconnect from the internet immediately: This prevents the virus from communicating with external servers, downloading additional malware, or stealing your data. Unplug your ethernet cable or turn off your Wi-Fi connection right away.
  2. Back up essential files to a clean external drive: Before attempting removal, save your most important documents, photos, and files to an external storage device. Scan these files on a clean computer before using them to ensure they’re virus-free.
  3. Update your operating system and security tools: Boot your computer in Safe Mode and update your operating system, antivirus software, and security definitions. This ensures you have the latest protection against current threats.
  4. Run a full system scan and remove the virus: Use your antivirus software to perform a comprehensive scan of your entire system. Follow the software’s prompts to quarantine or remove any detected threats. This process may take several hours but is essential for thorough cleaning.
  5. Reboot and perform a second scan: Restart your computer normally and run another full scan to ensure all threats have been eliminated. Some viruses can hide in the system memory and become detectable only after a reboot.
  6. Change all your passwords: Starting with the most critical ones such as banking, email, and social media, update passwords for all your online accounts. Be sure to use strong, unique passwords for each account and enable two-factor authentication.
  7. Review browser extensions and startup programs: Check your web browsers for suspicious extensions or toolbars and remove anything you don’t recognize. Also review your system’s startup programs and disable unfamiliar applications.
  8. Restore from clean backups if needed: If the infection is severe or you’re still experiencing issues, consider restoring your system from a clean backup created before the infection occurred. This is the most reliable way to ensure complete virus removal.

For comprehensive virus removal guidance, consult resources such as the Federal Trade Commission’s updated malware removal advice. Remember that prevention is always better than removal—keep your security software updated and practice safe browsing habits to avoid future infections.

→ Related: How to Wipe Out a Computer Virus

If you continue to see symptoms of virus infection, consider using reputable security software with real-time protection and automatic updates such as McAfee Antivirus, which can detect and remove threats that manual methods might miss.

Catching these signs early gives you the best chance to remove these viruses, and protect your computer and personal information. For additional guidance on computer virus removal and prevention, visit the Cybersecurity and Infrastructure Security Agency or the FBI’s Internet Crime Complaint Center websites for the most current security recommendations.

Tips to avoid computer viruses

Practicing universal precautions to prevent viruses or any kind of cyberthreats is the best solution. Universal precautions simply mean that you improve your security layers offline and online to enjoy browsing without worries. It also means you adopt good browsing behaviors to keep your computer safe, such as protecting your passwords and financial details and avoiding suspicious-looking websites.

Observing the most basic precautions will prevent many infections. Get started with this list of tips to protect your computer and personal data against computer viruses:

Update your system regularly

Viruses can usually find an entry point from an out-of-date browser, operating system, or software application. Make sure to update your operating system, browser, and installed programs because they will often include additional safety features.

Remain alert

Hackers and malware authors can use clickbait to lure you into clicking on a suspicious link to download the virus onto the user’s system. Always assume the worst when it comes to links and attachments from unknown or unsolicited sources.

Check the URL

Before accessing a website, always check the URL and the website privacy protocols by hovering your mouse cursor over the link. Legitimate and safe URLs will begin with “https” instead of just “http” as proof that they are encrypted and can protect sensitive information.

Install and use security features

Employ a multi-layered approach because no single security measure can stop all threats. Only trust software products with an established reputation, and always download applications directly from the main website of the company, brand, or product. You might want to consider the capabilities offered by McAfee’s family of security products such as McAfee Total Protection.

Final thoughts

Viruses are malicious programs that can infiltrate your system, disrupt your daily activities, and compromise sensitive information. As these cyberthreats continue to evolve, so will the tools and knowledge to remove them from your infected device.

However, prevention is always far more effective than recovery. With awareness and proactive measures, you can significantly reduce your exposure to viruses and protect yourself effectively. It comes down to maintaining strong digital hygiene: keep your operating system and applications updated, practise safe browsing habits with trusted security solutions, use trusted antivirus software, run regular antivirus scans, and back up important files. Equally important is practicing safe browsing habits such as avoiding suspicious links, attachments, and downloads, even if they appear legitimate.

Staying informed through trusted sources such as the CISA’s cybersecurity resources and the FTC’s online security guidance can keep you a step ahead of these emerging threats and you can confidently safeguard your data and devices in an increasingly connected world.