For millions of years, humans have coevolved with microbes that live within us, each shaping the other. Like all life on earth, microbes interact with their environment. Hundreds of different kinds of bacteria reside in and on the human body, including the skin, eyes, mouth, nose, and intestines, as well as specialized cells that serve a variety of functions. A person is a community of species, each habiting a specific environment within our body. Bacteria in the digestive tract break down the food that we consume. Without these useful microbes, we would be unable to digest our food. A zoo of microbes in the mouth protect against food bacteria, as well as viruses that can cause infection. Bacteria in breast milk help protect newborns. Useful viruses within our bodies inject their DNA into our reproductive cells which are then incorporated into the human genome where they are passed down from generation to generation. For example, retroviruses in the female placenta help protect against antigens from the father which are foreign to the maternal immune system. Without these viruses, the embryo would never reach maturity. Some viruses in our human genome have been there for millions of years. Useful bacteria and viruses reflect significant changes in the evolutionary history of all forms of life, including the human body.