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What is a virus?

A virus is a program that can replicate by attaching itself to other programs. The infected program must be executed for the virus to run. The virus might then simply replicate, or also do damage by immediately performing some harmful action.

A virus might be programmed to perform a negative action in the future, such as on a particular date or when some logic within the host program is activated.

A worm overloads memory or a hard drive space by replicating repeatedly.

A Trojan Horse is another type of computer infestation that, like the worm, does not need a host program to work. It actually substitutes itself for a legitimate program.

A logic bomb is dormant code added to software and is triggered by a predetermined time or event.

Viruses, Trojan horses , logic bombs and worms can occur in a combination such as when a virus gains access to a network by way of a Trojan horse. The virus plants a logic bomb written with application software on the network that sets off a worm when it executes.