The first computer worm didn't steal anything, encrypt anything, or demand a ransom. It hopped from computer to computer and printed a message: I'M THE CREEPER : CATCH ME IF YOU CAN.
Creeper
In 1971, Bob Thomas, a programmer at the research firm BBN, wrote a program called Creeper. It ran on DEC PDP-10 computers connected to ARPANET, the government-funded network that grew into the internet. Creeper would move itself from one machine to another, print its taunt on the terminal, and move on.
Then Ray Tomlinson, the same BBN engineer credited with inventing network email and picking the @ sign for addresses, tweaked it so it copied itself instead of just moving. That self-copying version is generally considered the first computer worm.
Reaper
Having made a worm, Tomlinson did the responsible thing and wrote something to kill it. Reaper traveled across ARPANET, found copies of Creeper, and deleted them. It's often called the first antivirus program. So the first antivirus was written by one of the people who made the first worm, which is either poetic or a business model, depending on how cynical you are.
Harmless, for once
Creeper didn't damage anything. At most a couple dozen machines could have been involved, and their operators were in on the experiment. It was researchers asking a genuinely new question: can a program move around a network on its own? The answer was yes, and everything since has been the unfriendly version of that answer.
The legacy
The Creeper-versus-Reaper idea inspired Core War, a programming game where people write little programs that battle for control of a simulated computer. And every worm since, from the ones that crippled networks to the ones still out there today, traces back to a polite one that just wanted you to try to catch it.