Our bodies contain trillions of bacteria that have evolved to live in humans. More than 38,000 different species live in the human digestive tract, and bacteria occupy at least 20 distinct niches on our skin. The microbes carried by each host are collectively called a microbiome. Humans have evolved to take advantage of the bacteria, and the bacteria gain advantage from us. Box 1-2 describes examples relating to obesity and pain. Some bacteria help humans digest food, whereas others protect from particular pathogens. For example,
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