In computer science, a virus is a program that infects and replicates intself inside a computer. The infected files then spread to other computers. Some are damaging and some are not.

Give me an example!

Viruses are responsible for everything from the common cold, to the flu, to warts, to AIDS, and even some forms of cancer. It is more effective to vaccinate people against viruses than to try to treat them after they have become infected.