A computer virus is.....

An executable file designed to replicate itself while avoiding detection. A virus may disguise itself as a legitimate program. Viruses are often rewritten and adjusted so that they will not be detected. Anti-virus programs must be updated continuously to look for new and modified viruses. Viruses are the number one method of computer vandalism.

A parasitic program written intentionally to enter a computer without the user's permission or knowledge. The word parasitic is used because a virus attaches to files or boot sectors and replicates itself, thus continuing to spread. Though some viruses do little but replicate, others can cause serious damage or affect program and system performance. A virus should never be assumed harmless and left on a system.