The Creeper worm, an experimental self-replicating program written by Bob Thomas at BBN Technologies, Cambridge, Massachusetts (originally Bolt Beranek and Newman), is generally considered the first computer virus.
"Creeper infected DEC PDP-10 computers running the TENEX operating system. Creeper gained access via the ARPANET and copied itself to the remote system where the message, "I'm the creeper, catch me if you can!" was displayed. The Reaper program was created to delete Creeper" (Wikipedia article on Creeper virus, accessed 01-18-2010).