Probiotics are capsules or pills with live microorganisms—almost always bacteria or yeast—that are supposed to confer health benefits once people swallow them. Some of my friends, including a woman who was recently treated for cancer and a man with persistent digestive issues, bought the pills at the recommendation of doctors. Others, aware of a lot of new evidence about the ways microbes in our guts influence physical functioning, bought them on their own. Many hope the bacteria will improve their overall gut health, a desire reflected in bottle labels that say things like “improved digestion” and “clinically studied.”