A virus is a small application that sticks itself into other applications. When the other application is run, the virus also runs, and it can copy itself into more applications. In this way, it spreads like a biological virus and can chew up enough memory to shut a computer down.

Viruses are written by people. Why? Who knows. Some virus writers are hackers seeking information or a larger network's computing strength. Many appear to be working hand in hand with spammers; some are looking for respect in the programming community or simply to leave a mark.