“The aim of cleaning, then, should be to reduce bacteria numbers—but not to zero. Even harmful bacteria can be good for us when the immune system uses them for training. A couple of thousand Salmonella bacteria in the kitchen sink are a chance for our immune system to do a little sightseeing. They become dangerous only when they turn up in greater numbers. Bacteria get out of hand when they encounter the perfect conditions: a protected location that is warm and moist with a supply of delicious food.”