A parasitic program written intentionally to enter a computer without the users permission or knowledge. The word parasite is used because a virus attaches to files or boot sectors and replicates itself, thus continuing to spread. Though some virus's do little but replicate, others can cause serious damage or effect program and system performance. Some recent viruses that spread via email replicated so quickly that world-wide internet and mail server performance was degraded. A virus should never be assumed harmless and left on a system.

The main purpose of a virus is to cause damage to your computer or the network. It is always wise to have an up-to-date anti-virus program installed and running on your computer whenever it is in use.

The principles of "Safe Computing" are:

1. Keep your antivirus software up-to-date. New viruses are being deployed daily. If you have a Windows computer, you are in a high risk group. Use McAfee's Antivirus software (available from the HelpDesk) to take advantage of automatic virus definition updates .
2. Run a virus scan on all files that you download from the internet, from friends, from any source other than your own computer. Do a full system scan monthly.
3. Do NOT open ANY email with an attachment that you are not expecting, or whose sender is unfamiliar to you. Even if you do not open the attachment, some of the newest viruses can infect your system if you simply open the email message!
4. Do NOT open email attachments or click on hyperlinks in email messages from unknown correspondants.

They can travel as email attachments, via floppy disks, through a network, and as downloads from cracker or non-authentic sites (rarely from authentic sites). They can also be transmitted through chat clients such as mIRC, PRICH and ICQ.