What is a computer virus? Here's how to spot signs of viruses and avoid them

• A computer virus is a type of malware that can replicate itself on your computer and spread to other devices.
• You might be infected with a virus if your computer suddenly starts running more slowly, crashes a lot, or does things like running unusual programs without your permission.
• Here's everything you need to know about viruses, including the major types, signs you're infected, and how to avoid them.

Now that personal computers have been a part of everyday life for about 30 years, computer viruses aren't as mysterious as they once were. Simply put, a virus is a kind of malware that has the ability to replicate itself. Once activated, it can install itself on a computer, infecting the PC and enabling the virus to continue to spread to other computers.

Depending upon its payload, the virus can have no other effect on the PC other than copying itself, or it can be designed to cause a wide variety of damages — anything from holding the files on the computer for ransom (this is called ransomware) to deleting files, crippling Windows, or turning the computer into a resource for hackers to conduct distributed denial-of-service (DDoS) attacks.

What you need to know about computer viruses

Viruses have been around for decades (the first virus was created in 1971) and is so synonymous with the concept of malware that some people refer to any malware as a virus. That's not really true — there are many kinds of malware, and viruses are just one.

Unlike other kinds of malware, a virus lies dormant until it's activated — it's unable to execute itself on its own. (This is in contrast to a worm, for example, which can execute on its own with no human intervention. That means you might accidentally download a virus to your PC, but it won't do anything until you run the file or open the document it's embedded in.

At that time, the virus' code is activated, it can execute its payload, which might include stealing passwords, emailing itself to the contacts in your address book, or taking over your PC in a ransom attack.

There are a number of different kinds of viruses you can be infected with. While this might seem unimportant — why should it matter since you're already infected — it's important to know since each virus can affect your PC differently. Here are the most common varieties:

• Boot sector virus: A boot sector virus can take complete control of your PC by infecting the part of the hard drive that has startup instructions.
• Polymorphic virus: This kind of virus is especially insidious because it can vary its code, making it hard for antivirus software to detect and remove. Modern antivirus software is pretty good at detecting polymorphic viruses, but is far from perfect.
• Web scripting virus: This virus specifically targets vulnerabilities in web browsers, but the virus doesn't stay there — it can be designed to affect all aspects of your computer.
• Macro virus: Thankfully less of a threat than it once was, macro viruses are written in the "macro" language used to create scripts within programs like Microsoft Word and Excel. When a document with an infected macro is executed, it can do many malicious things to the PC.
• File infector virus: These viruses have the ability to inject malicious code into other programs and documents.

How to spot signs of a virus

The symptoms of a computer virus can vary dramatically because not all viruses are engineered to do the same thing. But if you are concerned you are infected with a virus, here are some things to watch for:

Your computer's performance has suddenly changed

If your PC runs much more slowly than it used to, a virus can be using your PC's resources.

Your computer has started to crash frequently

Many viruses are poorly written and can cause a lot of unexpected crashes and failures.

Lots of pop-up windows

You might start to see unusual pop-up windows in your web browser or elsewhere. They might be requests to go to websites, or your browser might open windows to other websites without your permission. Ironically, many viruses also open pop-up windows asking you to install antivirus software, which is additional malicious software.

New programs are running on your PC

You might see programs you don't recall installing starting up with Windows.

Your email account is sending unrequested emails to your contacts

This is a sign the virus is trying to replicate itself through your contacts.

How to avoid viruses

The good news is that viruses pose a small shadow of the risk they once posed in the 1990s and 2000s. Thanks to dramatic improvements Microsoft made to Windows, large-scale virus infections are much less common than they were a decade or two ago, and few people ever encounter viruses when using computers for normal, routine tasks. Even so, it pays to be vigilant by following these tips:

Keep your computer updated

Make sure your computer's operating system is up to date with the latest Windows and security updates.

Use antivirus or anti-malware software

This can include the security software built into Windows 10 or a third-party antivirus app. Regardless of what you choose, one of the main reasons people don't have to worry about viruses today is because they're automatically protected by Windows and anti-malware software. Disable those protections, and you've thrown yourself back to 2005.

Don't click suspicious links or attachments

You've no doubt heard this advice before, but that's because it's so important: Never click anything you don't trust. That includes both links and attachments in email — if you don't know the sender, or if the email's legitimacy seems questionable — don't open anything inside it. The same is true for following links on websites of questionable quality.

Related coverage from Tech Reference:

• What is a computer worm? Here's how to protect yourself from the replicating malware

• Ransomware can encrypt your files and force you to pay money — here's how to avoid ransomware, or deal with an infection

• What is adware? How to keep your computer free of annoying or potentially harmful ads

• What is spyware? 5 ways to protect your computer from being infected

• What is a pop-up blocker? How to enable your web browser's pop-up blocker or disable it to access necessary pop-ups