Computer viruses are like real-life viruses: When they're flying around infecting every PC (or person) in sight, they're scary. But after the fact...well, they're rather interesting, albeit in a gory kind of way. With this in mind, TechWeb shamelessly present, in chronological order, the 10 most destructive viruses of all time

Melissa (1999) - Estimated Damage: 300 to 600 million dollars

On Friday, March 26, 1999, W97M/Melissa became front-page news across the globe. Estimates have indicated that this Word macro script infected 15 to 20 percent of all business PCs. The virus spread so rapidly that Intel, Microsoft, and a number of other companies that used Outlook were forced to shut down their entire e-mail systems in order to contain the damage.

The virus used Microsoft Outlook to e-mail itself to 50 names on a user's contact list. The e-mail message contained the sentence, "Here is that document you asked for...don't show anyone else. ;-)," with an attached Word document. Clicking open the .DOC file -- and thousands of unsuspecting users did so -- allowed the virus to infect the host and repeat the replication. Adding insult to injury, when activated, this virus modified users' Word documents with quotes from the animated TV show "The Simpsons."