Worm

A computer worm is a self-replicating program that is able to copy and spread itself without the help of any other program. Worms do this by exploiting security holes or policy flaws in software or operating systems, such as the automatic file sending and receiving features found on many computers. A worm can replicate itself very quickly from a single copy. The Code Red worm, for example, replicated itself over 250,000 times in approximately nine hours on July 19, 2001.

Worms can slow down network traffic, delete files on a system, or send infected documents by email. They can be also used to send spam and mount Denial of Service (DoS) attacks.