Computer Viruses are malicious programs that attack computer systems.
A virus is generally not regarded as a living organism, but sometimes described as (similar to) software. When the first self-replicating computer programs made the rounds, they were experiments or pranks; for most, the point was solely reproduction. An early computer worm was beneficent, but escaped control.
We distinguish Computer Viruses from computer worms by the profligate scale of replication, viruses generating a broadcast of copies rather than a chain of copies. The obvious points of analogy across both types of virus include that viruses are tiny, invading a host much greater in size and complexity, without an overt signal, and that viruses disrupt some process in the host. Neither computer nor biological virus necessarily does damage. In biology, self-replication is an end, not a means, making the damage a side-effect. In the modern computer virus, the end is likely to be the action of a payload of malicious code. Now the term “virus,” in both environments, connotes an intrusive and damaging force carrying dangerous baggage. —CACM, “Protecting Computers and People From Viruses“