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One of the labs in my highschool was for typing training, and it had something like 50 barebones PCs in it. Every day you'd sit down at a random machine, which booted from a 5.25" floppy. The autoexec.bat on the floppy let you log into the network where it ran another batch file from your user directory there.

As an experiment I wrote a piece of batch script that would append itself to both the network and local batch files if it wasn't already there (i.e if it was on the disk it would copy itself into your network account and visa versa), put it on one random computer, then forgot about it for a few days.

By the time I got around to checking again it had propagated itself to every computer in the lab and presumably every network account that had logged in during that time.