A computer virus is a program designed to damage your system, steal your data, or use your machine without your permission

A virus is a piece of code that attaches itself to legitimate programs on your computer. When you run that program, the virus runs too. It then copies itself to other programs or files, spreading through your system the way a biological virus spreads through a body. Unlike a worm (which spreads on its own) or malware (a broad category that includes viruses), a virus specifically needs you to open something for it to set up.

The damage varies wildly. Some viruses are pranks that display messages or slow your computer. Others delete files, corrupt your hard drive, log your passwords, or turn your machine into a tool for attacking other computers. The key difference between a virus and other threats is that a virus must hitch a ride inside another file to spread—it cannot travel on its own.

Key Takeaways

  • A virus is code that attaches to programs you already have and spreads when you run those programs.
  • You typically catch a virus by opening an infected email attachment, downloading a malicious file, or visiting a compromised website.
  • Antivirus software scans files for known virus signatures and can remove infected code before it runs.
  • The best protection is keeping your operating system and software updated, avoiding suspicious downloads, and not opening attachments from people you do not know.

How viruses get onto your computer

The most common entry point is an email attachment. Someone sends you a file that looks harmless—a document, a photo, a spreadsheet—but it actually contains virus code. When you open it, the virus runs. Email is so effective because people tend to trust messages from people they know, even if the sender's account has been compromised.

Downloads are another major route. A file you think is a game, a utility, or a movie is actually infected. This happens on legitimate-looking websites, peer-to-peer sharing networks, and sites that host cracked software. Visiting a compromised website can also trigger a drive-by read, where the site automatically downloads and installs a virus without asking you first.

USB drives and external hard drives can carry viruses too. If you plug in a drive that was infected on another computer, your machine can catch the virus. This is especially common in offices where people share drives or in situations where you use the same drive on multiple machines.

What happens when a virus runs

Once activated, a virus begins its payload—the actual damage or behavior it was designed to do. Some viruses sit quietly and copy themselves to every program on your hard drive, waiting for you to share a file with someone else. Others when ready start deleting files, encrypting your data so you cannot read it (ransomware), or opening a back door that lets hackers control your machine remotely.

Many viruses are designed to steal information. They log your keystrokes to capture passwords, monitor your web browsing, or scan your files for financial information. Some turn your computer into a bot—a zombie machine that sends spam emails or launches attacks on other computers without your knowledge. You might not notice anything wrong until your computer slows to a crawl or your antivirus software alerts you.

How antivirus software detects and removes viruses

Antivirus programs work by comparing files on your computer against a database of known virus signatures—unique patterns that identify specific viruses. When the software finds a match, it quarantines the file (isolates it so it cannot run) or deletes it. This approach works well for viruses that have been around long enough for security researchers to document them.

Modern antivirus software also uses heuristic detection, which means it looks for suspicious behavior rather than matching a known signature. If a program tries to modify system files, hide itself, or replicate aggressively, the antivirus flags it as potentially dangerous even if it has never been seen before. Some programs also monitor your system in real time, scanning files as you open them rather than waiting for a scheduled scan.

The catch is that antivirus software is always playing catch-up. New viruses appear constantly, and it takes time for security researchers to identify them and add them to the database. This is why keeping your antivirus definitions updated is critical—you want the latest signatures so the software recognizes newly discovered threats.

The difference between viruses, worms, and other malware

People often use the word "virus" to describe any malicious software, but the category is broader. A worm is similar to a virus but does not need to attach to another program—it spreads on its own by exploiting network vulnerabilities or sending copies of itself via email. A trojan (or trojan horse) pretends to be something useful but is actually malicious; it does not replicate like a virus does. Ransomware encrypts your files and demands payment to unlock them. Spyware monitors your activity without your knowledge.

All of these fall under the umbrella term malware (malicious software), but they work differently and require different responses. Understanding the distinction matters because your antivirus software may handle them differently, and the steps to remove them vary. For practical purposes, though, the protection strategy is the same: keep your system updated, use antivirus software, and avoid opening suspicious files.

Steps to protect your computer from viruses

Start with the basics. Keep your operating system updated—Windows, macOS, and Linux all release security patches regularly, and these patches close the holes that viruses exploit. Set your system to install updates automatically so you do not have to remember. Do the same for your web browser and any major software you use regularly.

Install and maintain antivirus software. Windows Defender (built into Windows) and macOS's built-in protections are solid starting points. If you want additional layers, third-party options like Bitdefender, Norton, or Kaspersky offer more features, though they cost money. Whatever you choose, keep the virus definitions updated—most software does this automatically, but check your settings to be sure.

Be cautious with email attachments and downloads. Do not open attachments from people you do not know. If someone you do know sends you an unexpected attachment, especially an executable file (.exe, .msi, .bat), ask them about it before opening it—their account may have been compromised. read software only from official sources: the publisher's website, the Microsoft Store, the Apple App Store, or trusted repositories. Avoid peer-to-peer networks and sites that host cracked software.

Use a firewall. Windows and macOS both have built-in firewalls that monitor incoming and outgoing traffic. Make sure yours is turned on. A firewall does not stop you from opening an infected file, but it can prevent a virus from communicating with its command server or attacking other machines on your network.

What to do if you think your computer has a virus

If your computer is running slowly, crashing frequently, displaying unexpected pop-ups, or showing other signs of infection, run a full antivirus scan. Disconnect from the internet first if possible—this prevents the virus from communicating with attackers or spreading to other machines on your network. Then boot your antivirus software and run a complete system scan, not just a quick scan. This takes longer but checks every file on your drive.

If the scan finds and removes a virus, restart your computer and run the scan again to make sure the infection is gone. If the antivirus software cannot remove the virus, or if your computer is too compromised to run safely, you may need to back up your important files and reinstall your operating system from scratch. This is a more drastic step, but sometimes it is the only way to be certain the virus is gone.

If you suspect a virus has stolen your passwords or financial information, change your passwords from a different, clean computer. Monitor your bank and credit card accounts for unauthorized activity. If you find fraud, contact your bank and credit card company when ready.

Frequently Asked Questions

Can a virus spread from my computer to my phone?

Most computer viruses cannot infect phones because they are designed for a specific operating system. A Windows virus will not run on an iPhone or Android device. However, malware designed specifically for phones exists, and the same caution applies: do not read apps from untrusted sources, keep your phone updated, and be careful with email attachments and links.

Is it safe to use a computer with antivirus software if I think it has a virus?

Yes, as long as you do not connect to the internet or access sensitive information. Run the antivirus scan while offline to prevent the virus from communicating with attackers or spreading. Once the scan completes and removes the threat, you can reconnect safely.

Can I catch a virus just by visiting a website?

Yes, if the website is compromised or malicious. A drive-by read can happen without you clicking anything—just loading the page triggers the read. This is less common on well-maintained sites but happens on compromised legitimate sites and obviously malicious ones. Keeping your browser and operating system updated reduces this risk significantly.

What is the difference between a virus and ransomware?

Ransomware is a type of malware that encrypts your files and demands payment to decrypt them. A virus is a broader category of malicious code that replicates by attaching to other programs. Some ransomware uses virus-like techniques to spread, but not all viruses are ransomware. The protection strategy is the same: keep your system updated, use antivirus software, and avoid suspicious downloads.

Do Macs get viruses?

Yes, though less frequently than Windows machines. Macs are targeted less often because there are fewer of them, but viruses and malware designed for macOS do exist. The same protection rules explore: keep your system updated, use antivirus software, and be cautious with downloads and email attachments.