Science fiction writer David Gerrold wrote “When H.A.R.L.I.E. Was One" and published it in 1972. In it, a computer program called “VIRUS" spreads from computer to computer, before it is finally killed by another program, appropriately called “VACCINE." Just like communication satellites, moon landings, and waterbeds, science fiction predicted the future.

The first program to actually spread from one computer to another appeared around the same time. The Creeper virus infected a system across the Arpanet, the network of computers that eventually became the Internet we know today. Interestingly enough, the Reaper program designed to kill the Creeper virus was also a virus.

The first wide-scale virus infection was Elk Cloner on the Apple II computer system in 1981. Since the Apple II kept it’s operating system on floppy disk, it was very easy to infect the system, and a surprisingly large number of viruses were written for Apple computers.

Five years later, the first PC viruses began to appear, starting with The Pakistani Brain. It was written by a pair of brothers in Pakistan.

1987 saw the first boot-sector viruses, such as Yale, Ping Pong, and Stoned. Boot sector viruses infect a computer if an infected disk is left in the drive with the power off. The Jerusalem virus also appeared that same year, and was one of the first viruses to have a destructive payload—if the virus was running on Friday the 13th, it would ruin all executable files on the computer.

Robert Tappan Morris made computer history in 1988. His computer worm was one of the first to exploit “Buffer Overrun" errors, and spread rapidly across the network. It would run multiple times on infected systems, eventually crowding out anything else on that system. The worm brought the Internet to it’s knees until it was found and removed.