Heck, even the operating system could reproduce in some sense. If the operating system on one machine is set up to regard itself as DHCP server, it would begin responding to DHCP requests in the local network. And in the DHCP response it can provide network boot instructions telling other computers to network boot a copy of the OS image of itself that it is serving. Any other computer that is configured to net boot could then boot the OS. Of course most computers are not set up to net boot out of the box. But on a network where they use net boot already you might be able to race their own DHCP server and get some of the clients to boot your OS image instead.