It seems that we must eventually learn to live in a world with untrustworthy replicators. One sort of tactic would be to hide behind a wall or run away. But these are brittle methods: dangerous replicators might breach the wall or cross the distance, and bring disaster. And, though walls can be made proof against small replicators, no fixed wall can be made proof against large-scale, organized malice. We will need a more robust, flexible approach … seems that we can build nanomachines that act somewhat like the white blood cells of the human immune system: devices that can fight not just bacteria and viruses, but dangerous replicators of all sorts.