Your body consists of about 10 trillion human cells and another 100 trillion noninfectious bacterial cells, most of them in your gastrointestinal tract but others on your skin, in your mouth, and throughout the rest of your body. These bacteria have been evolving since you were born to be better optimized to the unique environments of your various body parts. (Just as amazingly, according to a theory now accepted by most biologists, your cellular power plants, called mitochondria, are descended from free-living bacteria.) To these creatures, you are a garden, an island, a universe.