Debuted: July 2001
Origin: Unknown
Access method: Loophole in Microsoft's Internet Information Server software (now patched)
What it did: Replicated itself, sending copies to others. Also deleted files.
Scope: About 400,000 computers affected.
Cost: About $2.5 billion***
Postscript: Initial targets included White House websites. Followed weeks later by Code Red II, which disrupted business at some of America's largest companies.
*** VeriSign Inc. executive's testimony to U.S. House of Representatives telecom subcommittee, 2003