While a computer virus requires some sort of human intervention to be launched, such as opening an e-mail, a worm takes off on its own.

CNN: Sasser worm rips through Internet

Blaster was designed to exploit a known loophole in Microsoft's code that allows the worm to spread through network connections, rather than e-mail.

ECONOMIST: Target: Microsoft | The

In a cunning twist by the virus writers, an e-mail in wide circulation that purportedly offers a "fix" for the Sasser worm actually infects the user's computer with a different virulent worm, known as Netsky-AC.

CNN: Sasser worm rips through Internet