A worm is a program which reproduces itself over a network. The first significant worm was the Great Worm released into the wild in 1988. This accidental release infected thousands of systems and caused great, albeit temporary, damage to the network. Most of the programs which people refer to as "viruses" are actually "worms".

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Jargon dictionary entry
Viruses - Code Red Worm System administrators will remember the summer of 2001 as a strange time indeed. Code Red and Code Red II are some of the reasons.