Book excerpt: Trillions and trillions of microbial cells live on and inside your body. A small number of these microbes are unhealthy germs. But most belong on your body and perform essential jobs. Microbes help digest your food, protect you from dangerous germs, and help your body fight disease. Using techniques such as DNA sequencing, scientists are uncovering the many secrets of the human microbiome. Scientists are learning how the foods we eat and the medicines we take, such as microbe-killing antibiotics, can affect the bugs in our bodies. They are learning more and more about this system that keeps us healthy and how we can protect it in return.