A few years ago Tom Duff created a very persistent UNIX virus. At that point we had about 10-12 8th or 9th edition VAX 750s networked together. The virus lived in the slack space at the end of the executable, and changed the entry point to itself. When the program was executed, it searched the current directory, subdirectories, /bin, /usr/bin for writable, uninfected files and then infected them if there was enough space.