We are not alone. No matter how clean we are or how healthy we feel, we carry around on our bodies billions of microbes-microscopic one-celled organisms called bacteria and viruses. Although they cannot be seen, microbes hide under fingernails, lurk between teeth, and live in hair. There are more than six hundred thousand bacteria living on just one square inch of skin, and an average person has about a quarter of a pound of bacteria in and on his or her body at any given time. There are more microbes on a person's body than there are humans on Earth. Viruses and bacteria are responsible for some of the deadliest diseases in history, such as AIDS, the plague, and flu. And yet bacteria perform the most important roles in maintaining life on this planet. "They [bacteria] protect us and feed us," says Abigail Salyers...