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There’s yet another aspect to early exposure to microbes. We’ve grown up with the idea that dirt is bad. If you drop food on the floor, don’t dare to pick it up and eat it. Sanitize your kitchen and bathroom with “germ killers.” Filter your water. Purify your air. Sterilize baby bottles. Well, maybe all that attention to fastidious cleanliness isn’t serving us so well. Maybe our immune system needs the exercise of dealing with microbes.

Proponents of the “hygiene hypothesis” maintain that if our immune system is deprived of the targets it has evolved to deal with, namely microbes, it turns its weapons on whatever target is available, even if this target is not dangerous. That target may be a protein in peanuts or an ingredient in a perfume. Immune reactions often involve inflammation as the body rushes white blood cells to the site of a perceived attack by an intruder. Sometimes inflammation can become chronic and may even be implicated in diabetes, heart disease and stroke. Indeed, a study in the Philippines, where sanitation is not what we are used to here, showed that the more disease-causing microbes to which children are exposed, the lower their blood levels of a marker of inflammation known as “C-reactive protein” by the time they reach the age of 20. For example, having spent time in a place with possible exposure to animal feces during childhood significantly reduces C-reactive protein levels in later life. Somehow it seems that early exposure to germs reduces the risk of chronic inflammation. And such exposure may even reduce the risk of cancer.