Hoax
Hoaxes are messages that claim to be warnings of real virus threats. There are even hoaxes that warn of other hoaxes being infected by viruses. Computer virus hoaxes have been around for nearly as long as the first virus.
Hoaxes have only a single purpose and that is to spread to as many people as possible. Hoax messages generally include an admonishment to "forward this to everyone you know" and may even reference a seemingly legitimate source to gain credibility.
For a list of popular email hoaxes, see the Hoax Encyclopedia.