Often what people believe is a virus infection is, in fact, a worm program. This is not as serious, because worms do not replicate themselves. However, the damage caused by a worm attack can be just as serious as a virus, especially if it is not discovered quickly.
For example, suppose a worm program instructs a bankís computer to transfer funds to an illicit account. The fund transfers may continue even after the worm is destroyed. However, once the worm invasion is discovered, recovery is much easier because there is only a single copy of the worm program to destroy; the worm cannot replicate itself.
A well known worm
Simpsons
This is an AppleScript worm that is designed to propagate with MacOS 9.0 (or higher), and Microsoft Outlook Express 5.02. It arrives as an attachment to an e-mail message. When you run the attachment, it infects the local machine which is then used to propagate the virus.
This information and more can be found at http://virexhelp.com