Article from Tom Duff, August 1987
Executable files in the Ninth Edition of the UNIX system contain small amounts of unused space, allowing small code sequences to be added to them without noticeably affecting their functionality. A program fragment that looks for binaries and introduces copies of itself into their slack space will transitively spread like a virus. Such a virus program could, like the Trojan Horse, harbor Greeks set to attack the system when run by sufficiently privileged users or from infected set-userid programs.