25 years ago this month, a 9th grader from Pittsburgh, PA marched into history as being the author of the first recorded computer virus. It wasn't a malicious virus, really more of a prank to annoy friends and teachers.
The virus, called Elk Cloner, spread between Apple II computers via floppy disks (the 5.25" variety). When the computer was booted from an infected floppy, the virus would automatically start. Once infected, the system would then write a copy of the virus to any clean disk inserted in the drive. The only reason people knew they had it was because every 50th boot it would display a little "poem" and play a few more subtle, but equally harmless tricks, every 5th boot.
Slashdot has more on the virus, and a lengthy discussion. Check it out at the link below.