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The following are necessary characteristics of a virus:

In essence, a computer program which has been infected by a virus has been converted into a trojan horse. The program is expected to perform a useful function, but has the unintended side effect of viral code execution. In addition to performing the unintended task, the virus also performs the function of replication. Upon execution, the virus attempts to replicate and ``attach'' itself to another program. It is the unexpected and generally uncontrollable repliction that makes viruses so dangerous.

Viruses are currently designed to attack single platforms. A platform is defined as the combination of hardware and the most prevalent operating system for that hardware. As an example, a virus can be referred to as an IBM-PC virus, referring to the hardware, or a DOS virus, referring to the operating system. ``Clones'' of systems are also included with the original platform.
Thu Mar 10 15:32:44 EST 1994