While there is no widely-accepted definition of the term computer virus, the following loose definition should suffice: A computer virus is executable code that, when run by someone, infects or attaches itself to other executable code in a computer in an effort to reproduce itself. Some computer viruses are malicious, erasing files or locking up systems; others merely present a problem solely through the act of infecting other code. In either case, though, computer virus infections should not go untreated. · Closely related to computer viruses are Trojan Horses and worms. A Trojan Horse is a program that performs some undesired yet intended action while, or in addition to, pretending to do something else. One common class of trojans are fake login programs - collecting accounts and passwords by prompting for this info just like a normal login program does. Another is a disk defragger that erases files rather than reorganizing them. A Trojan Horse differs from a virus in that the former does not attempt to reproduce itself. A Worm is just a self-propagating virus. The Internet Worm from November '88 is a famous example.

See also Why virus is so called ? and Virus creation !