Virus

Virus - A program created to replicate itself inside a program, it may make the program operate incorrectly or corrupt a computer's memory.

Viruses usually come from a Torrent, or some sort of 'Free' download that you might get.

Layman's Terms A virus is a program that can copy itself into programs, this may cause the program to run in a wrong manner.

Example: One of your Programs that you Torrented doesn't work well, or from that day on, a different program starts to act all weird, you may be inflicted with a virus