There is a vast city hidden inside your body. You might not be directly aware of its inhabitants, but it’s quite likely you owe your health to them. These inhabitants are microbes, of course. Hundreds of species of them inhabit us, and the term “microbiome” refers to their collective genes. Even though scientists are just beginning to map it, many researchers believe the microbiome affects our health—and when it’s unbalanced it predisposes us to numerous autoimmune diseases. Is our sterile Western environment—which has rid our bodies of many kinds of bacteria—to blame for this?