A computer virus is generally a small program that is able to attach itself to another program, document or media file. It usually knows how to jump from one file to another and is often adept at hiding itself in existing files.

Viruses are a relatively common and well-known threat, and there are software packages that are able to deal with them very effectively. These packages must be constantly updated, as viruses can often mutate and evolve into new forms; the AntiVirus manufacturers constantly monitor these new viruses and include them in updates to the definitions embedded in their products.