Short of a complete system breakdown, there is nothing computer users dread more than a virus infection. Viruses latch onto your machine, spread to other people’s computers, and if you happen to catch a virulent strain, you may get that system breakdown in the bargain.

People who already have a reliable antivirus program installed may be tempted to feel complacent, thinking they’ve safeguarded their system from any would-be intruders. And yet, when a virus pops in and wreaks havoc on their computer, they find themselves at a loss—how did it even get in there?

To understand how viruses can penetrate our systems, we have to first understand how they work. The word virus is actually an umbrella term for several kinds of threats. A more complete word to use would be malware—that is, malicious software that secretly perform unauthorized and even criminal actions on your computer.

A virus hitches onto a host and spreads copies of itself to other computers. It is somewhat different from worm, which does not need a host program and spreads itself through a network by exploiting loopholes in security systems. A trojan, like its name implies, is a program that pretends to be useful but actually contains harmful code. Spyware, while unable to copy themselves, can capture information about yourself or your habits and exploit that data however way their creator wants. Finally, adware, while not necessarily harmful, can still annoy you by continually flashing advertisements on your screen.

Based on the different kinds of malware described, you can already get clues on how a virus can sneak inside your system. For clarity, here’s a rundown of the different means malware can infect computers, and also how to keep that from happening.

Email attachments – Downloading an infected file attachment is one of the most common ways to get a virus. This can be pretty insidious as well, as the email usually pretends to be something funny, important or useful, and may even come from someone you know. Just keep in mind that viruses have been known to hijack email accounts and send themselves to all the entries in the user’s contact list. Bottom line: Don’t open email attachments from unknown senders. Even if you do get an unexpected email from someone you know, first confirm that they did in fact send you that message before opening the attachment.

Security Vulnerabilities – Programmers do make mistakes and oversights with their work. Viruses may also exploit the large number of security loopholes found in operating systems and their bundled programs. Vulnerabilities in such programs as Internet Explorer and Microsoft Office may allow malware to perform unwanted actions, like control your system or steal private information. Have your computer download regular updates and patches to fix these security holes.

USB drives, DVDs, memory cards and other removable media – Modern viruses know how to detect an inserted media device and silently copy themselves onto it. They may even hide their files on the device, allowing them to infect and re-infect each computer they’ve been attached to. If you tend to share your flash drives and memory cards with others, you are at high risk. Regularly scan your media devices for any unwanted hitchhikers.

Peer-to-Peer programs – File-sharing programs are not spared the attention of malware programmers. Downloading files from P2P applications opens you up to the risk of spyware, adware, trojans and viruses. This especially true for pirated software that require a “crack” or a modified executable program to work. Avoid downloading software, music and other files through P2P. You may even avoid a hefty fine in the process.

Websites and Online ads – Another common means of infection, downloading programs or files from websites can leave you vulnerable to viruses. Mostly these sites or ads pretend to be useful, like an online scanner warning you that you may already be infected by a virus. If you have to download or install anything from a website, make sure that your virus scanner is active, that the site is a trusted source and that you have read their terms and conditions. If they don’t have such a document, you have no business downloading anything from them.

Outdated antivirus programs – Antivirus programs are powerful tools against malware, but they’re only as good as their last successful update. If for some reason you can’t update them (say you don’t have internet access or you can’t obtain a license), you’ll be leaving a gaping hole in your system’s defenses. Have your antivirus programs updated regularly, and avoid attaching removable media or downloading files until you’ve done so.

These are the many means you can get infected by a virus. While having a good antivirus program helps, your first and best line of defense is awareness. Learn to recognize and avoid these pitfalls and you can keep your system, as well as your whole network, safe and sound. To stay virus-free, read up on the three basic steps for computer virus prevention.