A computer virus is a type of computer program that is designed to hide in the background and replicate itself by attaching itself to existing programs or parts of the operating system. There are now more than 22,000 known computer virus in circulation throughout the world and more are being discovered every day. Most folks lump together worms, trojan horses and viruses under the term "virus", but they are actually different things. Here is a site with easily understood definitions for each.

Even a simple virus is dangerous because it can quickly use all available memory and bring the system to a halt. Some infected code will perform malicious deeds, like deleting or scrambling users' files or their entire system, disabling certain functions, causing erratic system behavior, or transmitting itself across networks and bypassing security systems.

Users can unknowingly transmit a virus from one system to another by copying infected program files from one machine to another or by using an infected program downloaded from the Internet. One of the most prevalent means of transmitting viruses is via email. Often times, we receive an email that looks like it is something we want, but instead it is a trojan horse containing harmful code. Even if the file comes from a friend, you still must be sure what it contains before opening it, because often, a trojan horse will automatically access your email address book send itself to everyone you know without your knowlege! When in doubt, ask the sender before openning an unexpected attachment, and scan it with a fully updated anti-virus program.Never download an executable program just to "check it out" - if it's a trojan, the first time you run it, you're already infected!