Human beings swarm with bacteria, viruses, and fungi — trillions and trillions of them, inhabiting virtually every part of our bodies. Known collectively as the microbiome, they play a vital role in keeping the immune system strong, synthesizing nutrients, maintaining heart health, and so much more. But when these synergistic communities of microbes are disrupted, we become more susceptible to infections and disease. Recent breakthroughs in manipulating the gut microbiome have opened new therapeutic pathways, including fecal microbiota transplants — in which healthy stool samples are infused into a patient’s body during a colonoscopy. Gastroenterologist Ari Grinspan explains their therapeutic role.