Viruses are not a significant problem with Linux, but are with DOS and Windows (95, 98 and NT--there is at least one NT-specific virus that I was able to find).
There are virus development kits out there on the Internet tht anyone can download and use to create their own viruses with their own payload. Payload is the term used to describe the code in the virus that runs once the virus has been delivered. This can be anything from displaying a simple message to deleting all your files.
If you're running Windows of any sort install virus detection software. F-PROT and Virex are available from ACSU for all staff and students; just download from their web site. They also have regular signature-file updates.
ACSU's home page is at: