The term “microbiome” is shorthand for the vast and still largely unexplored worlds of bacteria, viruses, fungi and other microorganisms that inhabit every corner of the planet. Bacteria form tiny ecosystems side by side with our own cells on our skin, in our mouths and along our airways and digestive tracts, as well as on all the surfaces we interact with—including our homes, workplaces, and hospitals, and the air, water, and soil.