It's a piece of software code designed to invade computers and networks through e-mail or Internet connections and attach to files or programs or the hard drive, replicating itself. Viruses often present taunting messages, destroy files, or make the computer crash. Example: LoveLetter, in 2000, caused $8 billion in damage globally.WORM
A worm can spread itself automatically over the network from one computer to the next. Users need not click on e-mail or open a program to get infected. Examples: In early August, Blaster infected more than 1 million computers. Welchia, an antidote to Blaster, crashed computers and clogged networks, too.TROJAN HORSE
This is malicious code that masquerades as a benign program. These bugs don't replicate, but they are used to open "back doors" in computer systems to allow a hacker to take control or steal confidential data. Example: In July, 2000, QAZ helped hackers view secret source code at Microsoft.BLENDED THREAT
Viruses that try different ways of infecting and spreading, including basic virus, worm, and Trojan Horse techniques. Many attempt to use peer-to-peer file-sharing and instant messaging for distribution. Example: SoBig, which hit in mid-August, infected one out of two e-mails traveling on the Internet at its peak.
Data: Symantec Corp.