Every one of us teems with trillions of microorganisms: Streptococcus, Flavobacterium, and thousands of other kinds of bacteria and fungi. Some of these microbes are what we call "germs," but many more of them are actually good for us. Discover the health benefits of having aliens inside us, and see how those benefits are being threatened by our modern, sterile lifestyles. Then follow Dr. Dominguez-Bello on her expedition to the Amazon, where DNA samples of a remote group show how drastic the microbial imbalance in the Western world has become.