Paradigm shift is an overused term. Properly, it refers to a radical change of perspective on a topic, such as the move from the physics of Newton to the physics of Einstein, or the introduction of plate tectonics into geology. Such things are rare. Something which history may come to regard as a true paradigm shift does, however, seem to be going on at the moment in medicine. This is a recognition that the zillions of apparently non-pathogenic bacteria on and in human bodies, hitherto largely ignored, are actually important for people’s health. They may even help to explain the development of some mysterious conditions.