A computer worm is a standalone malware computer program that replicates itself in order to spread to other computers. Often, it uses a computer network to spread itself.
The worm uses holes in a target computer’s security. Unlike a computer virus, it does not need to attach itself to an existing program. Worms almost always cause at least some harm to the network, even if only by consuming bandwidth, whereas viruses almost always corrupt or modify files on a targeted computer.
On November 2, 1988, Robert Tappan Morris unleashed what became known as the Morris worm, disrupting an estimated 10% of the computers then on the Internet.