Briefly, three different items are commonly called a virus.

A true Virus is a program that attempts cripple your computer by "infecting" key files and spreading itself on to others. It might change critical files or performing unwanted functions like erasing your hard drive. NATAS (Satan backwards) is a particularly nasty virus.

A Trojan Horse will leave your computer accessible for other people to use (a common Trojan Horse is Back Orifice). Once a Trojan Horse is installed they may "bounce" through your machine as a way to cover their tracks and do other damage. A Trojan Horse commonly gives access to all the information stored on your hard drive, so they can read what you have, or use you as their storage server.

A Worm will travel from system to system and gather information and send it on, also doing collateral damage along the way. The ILOVEYOU scare was basically a worm.

One of the designs of a virus is not only to infect you, but to spread itself. Some have been know to read an address book, and e-mail itself to everyone in the book (prettypark) or attaches itself to every outgoing mail (Happy99). So, simple steps to be safe include:

• Do not open an email attachment if you do not know the sender, or the message text or attachment name seems odd.
• Scan all attachments and files you receive with and frequently updated anti-virus program.
• Do not download from sites you are unfamiliar with or sites that may be questionable.