Wat kinds of computer viruses are there and what do they do?

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many types of viruses are:

Trojan horses

A Trojan horse is just a computer program. The program pretends to do one thing (like claim to be a picture) but actually does damage when one starts it (it can completely erase one's files). Trojan horses cannot replicate automatically.

Worms

A worm is a piece of software that uses computer networks and security flaws to create copies of itself. A copy of the worm will scan the network for any other machine that has a specific security flaw. It replicates itself to the new machine using the security flaw, and then starts replicating.

E-mail viruses

An e-mail virus will use an e-mail message as a mode of transport, and usually will copy itself by automatically mailing itself to hundreds of people in the victim's address book.

Computer viruses are called viruses because they share some traits of types of biological viruses.

A computer virus will pass from one computer to another like a real life biological virus passes from person to person. For example, it is estimated by experts that the Mydoom worm infected a quarter-million computers in a single day in January of 2004. In March of 1999, the Melissa virus spread so rapidly that it forced Microsoft and a number of other very large companies to completely turn off their e-mail systems until the virus could be dealt with. Another example is the ILOVEYOU virus which occurred in 2000 had a similarly disastrous effect.