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

But the novel worm spotted recently by Spamhaus routes junk via the mail servers of the net service firm that infected machines used to get online in the first place.

BBC: PC hard drive, Eyewire

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