Pervading Animal and Elk Cloner

In January of 1975, John Walker discovered a new way of distributing his Univac game files and inadvertently wrote the world's first computer virus.

The game was called ANIMAL, a self-learning variation of 20 questions which asked you to "think of an animal." Tired of mailing several copies of the game on magnetic disks out to curious geeks, he started work on a clever way to distribute the ANIMAL executable to every Univac system in the world. He coded a subroutine called PERVADE, which could be called by any program and quietly copied itself into every directory that the current user had access to. It eventually made its way into software distribution tapes from Univac themselves. The entire story, with assembly source code for ANIMAL and PERVADE, is available from Walker's homepage.

Written in 1982 for the Apple II, the first microcomputer virus was also (mostly) benign. In 9th grade, Richard Skrenta, Jr. wrote a program called Elk Cloner that stayed resident in system memory after its disk was removed, but would later copy itself to any new disk inserted into the drive. Elk Cloner counted the number of times the infected disk was booted from, and on the fiftieth boot, the screen would display a little poem. The source code for Elk Cloner is available from Rich's website.

You might know Rich from his later work; he went on to co-found NewHoo, later renamed to DMOZ (aka the Open Directory Project).