Creeper was possibly one of the first programs that resembled a computer virus. It ran on the Tenex operating system and spread through the ARPANET, a predecessor of the current Internet. Creeper caused infected systems to display the message "I'M THE CREEPER : CATCH ME IF YOU CAN.". Not only was Creeper the first rogue mobile program, but another version of it was created that could be considered the first worm, as it would spread, leaving copies of itself. Yet another similar program was created to fight the Creeper infections, named Reaper, which was the first nematode.
When run, the Creeper program will start to print a file, but then stop, find another Tenex system, open a connection, pick itself up and transfer to the other machine (along with its external state, files, etc.), and then start running on the new machine. The program rarely, if ever actually replicated itself, rather it "jumped" from one system to another.
It is uncertain how much damage Creeper actually caused. Some sources claim that Creeper replicated so many times that it crowded out other programs. The extent of the damage is unspecified. Most sources say the worm was little more than an annoyance.
A version of Creeper was allegedly created by Ray Tomlinson that could replicate itself. Another program called "Reaper" was created to delete Creeper. It is presumably similar in most if not every way, except that it looks for a Creeper infection and deletes it.
In addition to having the first worm in history, Tenex is the OS which saw the first email programs, SNDMSG and READMAIL, in addition to the use of the "@" symbol on email addresses. The first email program, email message and email itself were invented by Ray Tomlinson, the creator of the replicating Creeper. Bob Thomas and Ray Tomlinson were a employees of Bolt, Beranek and Newman (now known as BBN Technologies), which developed these and many other early internet programs and protocols.
As Creeper's run through the DARPANET was ages ago, a complete description of this program may require some real Internet archeology (actually visiting places where these things happened to find information). The information is horribly incomplete, its accuracy is uncertain and sources often conflict on some details. No samples or source codes are currently available for study.
Most sources claim Creeper ran through the DARPANET in the early 1970's, but David Sims's article in Wired says it was in the late 1960's. Some articles are a bit more exact, but they are no more consistent.
Viruslist.com. History of Malware, 1970s
Thomas M. Chen. Department of Electrical Engineering SMU, The Evolution of Viruses and Worms (PDF)
Alexander Dewdney. Scientific American, (1984, v.250, N 5, pp. 15–19) In the game called Core War hostile programs engage in a battle of bits
Dave Crocker. Living Internet, "Email History".
John Metcalf. corewar.co.uk Creeper and Reaper
Corinne Cullen Hawkins. A Software Bestiary: Computer Parasites & Remedies
Ed Skoudis, Lenny Zeltser. Malware: Fighting Malicious Code, "A History of Worms". Prentice Hall, 2003.11.07. O'Reilly Safari Online Books.
David Sims. Wired, Privacy's Yin and Yang 2000.07.21
Sean Kazen. E-Articles, A Brief History of Worms
Ray Tomlinson. Email Home.