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. Obviously a computer virus isn't anything like an actual human cold or illness. 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, opening an unsafe email attachment, or through peer-to-peer file sharing. Malware is the blanket 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 phone or laptop might be infected with malware include ads that pop up all the time, odd system alert messages, or a slower-running computer (although, sometimes that's just a sign of old age). If you think your machine might be infected, the best thing to do is run some legitimate antivirus software such as Webroot, McAfee, or 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, 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, too.