The very first computer virus is now 40 years old. First appearing in 1971, a virus known as "The Creeper" popped up on infected computers, displaying the message "I'm the creeper, catch me if you can!"
PhysOrg provides a bit of background on the Virus:
Early viruses were simply an annoyance, done mostly to prove that the designer could. Current models are usually designed with another type of crime in mind, usually identity theft or creating a botnet. The first virus to really exploit the commercial potential of the virus was the Melissa, which ran rampant in 1999, and the first virus to make botnets came about in 2005. It was named MyTob.