Bacteria are microscopic, single-celled organisms. They are among the earliest known life forms on earth. There are thousands of different kinds of bacteria, and they live in every conceivable environment all over the world. They live in soil, seawater, and deep within the earth’s crust. Some bacteria have been reported even to live in radioactive waste. Many bacteria live on and in the bodies of people and animals—on the skin and in the airways, mouth, and digestive, reproductive, and urinary tracts—without causing any harm. Such bacteria are called resident flora, or the microbiome. There are at least as many bacteria in our resident flora as there are cells in the body. Many resident flora are actually helpful to people—for example, by helping them digest food or by preventing the growth of other, more dangerous bacteria.