Your body teems with them—100 trillion microbes in your gut, lungs, mouth, and skin. Your home swarms with them—in toilets and sinks, on tables and chairs, in the carpet, and on your dog. Even the ground on which you stand abounds with countless bacteria, fungi, protozoa, algae, and viruses—all microscopic, all part of a community of organisms interacting with one another and the environment. These communities and the environments with which they interact are known as “microbiomes,” and our growing understanding of them is changing the way we treat diseases, grow crops, and create everyday products.