Creeper - The First Computer Worm (March 15, 1971)

The very first computer virus, known as Creeper, was an experiment, and not intended to be malicious.  The virus had two functions: self-replication, and displaying a message to anyone using the computer system.
Pictured Above: A portion of a DECsystem-1090 showing the KL10 CPU and MH10 memory cabinets.  This is the same type of system which was vulnerable to infection by Creeper in the early 1970's.  Image by Michael L. Umbricht, The Retro-Computing Society of RI. Distributed under CC-BY-SA-2.5 license.

This worm spread across the ARPANET in the early 1970's.  Interestingly enough, the worm's only impact on the end user was display of the following message:

I'M THE CREEPER: CATCH ME IF YOU CAN
Creeper would jump from system-to-system, attempting to remove itself from the previous system.  Ultimately, a second worm, named Reaper, was created with the purpose of hunting down and removing installations of Creeper.  Reaper can be considered the first "Antivirus" program in computing history.

It's important to take note that this wasn't a network of home PCs.  The ARPANET was a network of some of the first networked computer systems, sponsored by the Department of Defense's "Defense Advanced Research Projects Administration" (DARPA).

Creeper infected systems running the TENEX operating system, a very early command-line OS.  The worm would perform the following actions:
1) Display the message "I'm the creeper: catch me if you can"
2) Begin to print a file
3) Stop printing the file
4) Locate another TENEX system on the network
5) Transfer itself to the new TENEX system
6) Start running on the new TENEX system
7) Remove itself from the originating TENEX system

It is believed that the worm did very little, if any damage, and was merely an annoyance more than anything.

Editor's note: There is conflicting data regarding the official date of "Creeper".  Some sources claim March 15, 1971 while other sources claim March 17, 1971.

References:
First computer virus of Bob Thomas - http://history-computer.com/Internet/Maturing/Thomas.html

Creeper, the first computer virus, is 40 years young today - http://www.engadget.com/2011/03/17/creeper-the-first-computer-virus-is-40-years-young-today/

Timeline of Notable Internet Pioneers and Contributions - http://www.juliantrubin.com/schooldirectory/internet_milestones_pioneers.html

The Virus Turns 40 - http://phys.org/news/2011-03-virus.html