[StanislavPetrov]

My first experience with an MS-DOS virus was in the late 1980s with the Jerusalem virus. We wanted to play Nethack during programming class (PASCAL) in the computer lab, and so brought a disk with a downloaded version (1200 baud) from home to install on the school's computers. Needless to say, someone had an infected copy, and before long it spread to most of the machines. It would cause weird things to happen like little blue rectangles to randomly appear. At that time, nobody was familiar with viruses (the computer teacher was actually a math teacher picking it up on the fly), and so the computers stayed infected for months until someone figured it out.