A computer virus is defined as “a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes.” In essence, a virus is an uninvited intruder to your computer system that has one of two purposes: to be a nuisance or to be destructive.
All computer viruses are man-made. A simple virus that can make a copy of itself over and over again is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems.
Since 1987, when a virus infected ARPANET, a large network used by the Defense Department and many universities, many antivirus programs have become available. These programs periodically check your computer system for the best-known types of viruses.