You’re more microbe than human, at least by the numbers. The human body has about 37 trillion cells, but it’s home to many more microbes—the gut alone has 100 trillion of them. Swimming and squirming inside your tummy are more than 1,000 species of bacteria, as well as a host of fungi and viruses. Together, they make up your gut microbiome, a unique-to-you community of tiny organisms that helps you draw energy from food and keeps your immune system running.