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Worms, in contrast, usually do not require any human intervention to spread. That means they can travel at the breakneck pace of computers themselves. A worm's danger lies in its speed: when it multiplies, it often generates enough traffic to crash internet servers. The most popular worms today are 'mass mailers' which attack a victim's computer, swipe the addresses out of Microsoft Outlook (the world's most common email program) and send a copy of the worm to everyone in the victim's address book. These days, the distinction between worm and virus is breaking down. A worm will carry a virus with it, dropping it on to the victim's hard drive to do its work, then emailing itself off to a new target.
The most ferocious threats today are 'network worms', which exploit a particular flaw in a software product (often one by Microsoft). The author of Slammer, for example, noticed a flaw in Microsoft's SQL Server, an online database commonly used by businesses and governments. The Slammer worm would find an unprotected SQL server, then would fire bursts of information at it, flooding the server's data 'buffer', like a cup filled to the brim with water.
Once its buffer was full, the server could be tricked into sending out thousands of new copies of the worm to other servers. Normally, a server should not allow an outside agent to control it that way, but Microsoft had neglected to defend against such an attack. Using that flaw, Slammer flooded the net with 55 million blasts of data per second and in only 10 minutes colonised almost all vulnerable machines.