You’re just browsing the internet when a half-dozen ads sneak onto your screen. Or, every time you start your computer, a pop-up or two offers a confusing message with only the option to hit “Okay.” Things like this used to be limited to PCs, but now they’re cropping up on Macs and mobile devices, too: You’ve got a computer virus.

But what, exactly, is it?

A virus is a type of malware — a piece of software designed to damage or disable a computer. You can accidentally download malware a lot of different ways: By clicking an ad on an insecure website, downloading an app from outside the App Store or opening an unsafe email attachment. Malware is the term for any kind of bad (malicious) code.

A virus generally refers to malware that self-replicates and spreads to different computers (without you knowing). Malware and viruses may erase parts of your computer’s code and/or rewrite it, or search it for personal information such as bank or log-in credentials.

As we mentioned before, signs your PC, laptop or phone might be infected with malware include ads that pop up all the time, odd system alert messages, or a slower-running computer. If you think your machine might be infected, the best thing to do is run some legitimate antivirus software such as Kaspersky Antivirus and follow its instructions.

The best way to protect yourself against malware is to keep your computer (and your phone) up to date on software. Plain and simple. Malware works by identifying errors or loopholes in your computer’s security (this is why it is highly recommended that anyone still using Microsoft XP move to a more updated operating system as XP no longer has any security updates), and with each new update, old holes are “patched” so that attacks targeting them won’t work anymore. It also helps not to open attachments or emails from unfamiliar contacts — that goes for text message attachments and, even more recently, WhatsApp links from unknown contacts.