The Sobig virus, spread by e-mail, has infected millions of computers all over the world.
The virus doesn't actually damage computers, but fills up e-mail inboxes with loads of unwanted mail.
Sobig is quite easy to stop, as it relies on fooling people into opening an attachment rather than using any techno-trickery.
A spokesman for a web security firm said "We haven't seen anything spread so fast."
The web has been infected by two other big viruses recently.
The MS Blast worm tried to attack the Microsoft site, and the Nachi worm was such a big problem that it actually caused Air Canada to cancel some of its flights