There are three ways by which you can contract a computer virus.

• The first is by opening a program that contains a virus inside. These are called Trojan Horse viruses because they trick you into loading themselves by masquerading as a useful program. They must be activated by opening the program before they can infect your system and often arrive as attachments in email.
• The second way is through a vulnerability in your web browser. Loading a web page that takes advantage of these sorts of flaws can load a virus without your knowledge.
• The final method is through a vulnerability in your operating system. A network-aware virus, or "worm", can use these holes to infect your computer without having you open any programs or go to a web page.