For every cell in your body, there's another tiny single-celled creature that also calls your body home. Far from being germs we should eradicate, these ancient friends allow us to digest food, breathe air, and fight off disease. They were here long before us and will undoubtedly remain long after we're gone. They are our microbiome, and after eons of cohabitation, we are finally getting to know one another better. Of course, we aren't always the best of neighbors. Autoimmune diseases, allergies, depression, and Alzheimer's may be diseases of an unhappy microbiome