The Morris Worm is widely regarded as the first fully recognised worm to attack and affect the internet. It spread throughout computers, most of which were located in the United States and utilised weaknesses within the UNIX system called Noun 1 and regularly multiplied itself.
Although it was very much in the early days of cyber attacks, the Morris Worm was able to slow down computers so that they essentially became useless. It was developed by Robert Tapan Morris, who claims he was just trying to use it to determine the size of the internet. As a result though, he became the first individual to be convicted using the US Computer Fraud and Abuse Act.