The first computer virus to appear on the ARPANET was Creeper. Created by Bob Thomas at BBN in 1971, Creeper was an experimental self-replicating program that moved between DEC PDP-10 computers running the TENEX operating system.
Creeper was not designed to steal information or damage files. After reaching a host, it displayed the message “I'M THE CREEPER: CATCH ME IF YOU CAN” and attempted to continue to another machine. Ray Tomlinson later modified it so that it copied itself rather than simply moving.
The program is often called the first computer virus, although modern security terminology more precisely classifies it as a computer worm because it spread across a network. That distinction does not change its historical importance: Creeper demonstrated network self-replication years before malicious malware became common.
In 1972, Tomlinson created Reaper, widely regarded as the first antivirus program, to find and remove Creeper from ARPANET-connected systems.