[Viruses] aren’t self-sufficient, or even fully alive; they always need to commandeer the cells of an already-existing host in order to reproduce. A virus is nothing but DNA or RNA encased in a protective sheath; that is to say, it is a message—encoded in nucleic acid—whose only contentis an order to repeat itself. When a living cell is invaded by a virus, it is compelled to obey this order. Here the medium really is the message: for the virus doesn’t enunciate any command, so much as the virus is itself the command. It is a machine for reproduction, but without any external or referential content to be reproduced.62