Elk Cloner is widely regarded as the first computer virus to spread outside its creator's computer. Richard Skrenta wrote it in 1982 for Apple II systems.
The virus spread through infected floppy disks. It attached itself to the disk's operating-system program and displayed a short poem after a specified number of boots. This made it more than a program that merely copied itself on one machine.
Elk Cloner was not a major destructive threat, but it demonstrated how removable media could carry malicious code between computers. It predates the IBM PC era and the large email-based virus outbreaks that became common later.
It is sometimes confused with Brain, a 1986 boot-sector virus often called the first IBM PC virus. Elk Cloner's historical distinction concerns early virus spread on Apple II computers.