A virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Most viruses can also replicate themselves and spread to other computers.

Viruses are an unfortunate fact of life in the computer world, and they can travel across any means of connecting computers, like modems and networks. The best thing to do is to use anti-virus software.

However, you can't catch a virus just from reading an e-mail message. Pranksters periodically send out "warnings" about just such a thing, like the notorious "Good Times" virus hoax. File attachments can carry viruses, but not the actual e-mail itself, so just don't download file attachments from anyone you don't know or trust.