Elk Cloner virus

Elk Cloner is one of the first known microcomputer viruses that spread "in the wild," i.e., outside the computer system or lab in which it was written. It attached itself to Apple DOS 3.3 operating system and spread by floppy disk. It was written around 1982 by a 15-year-old high school student named Rich Skrenta. It was originally a joke, created and put on to a game.

from Wikipedia