A probiotic is a food, drink, or pill that contains living bacteria — and sometimes yeasts — which we eat on purpose because they may help the gut. Yoghurt and little health drinks are sold this way. The word can also mean the use of those foods to try to improve health. As a describing word: probiotic yoghurt. The idea is “bacteria for you,” not “bacteria that make you sick.” Though actually, which bacteria help which people is still being worked out, so the claims on the packet are often bigger than the proof. Do not mix it with an antibiotic, which fights bacteria, or with a prebiotic, which is food for the good bacteria — fibre they can eat — not the bacteria themselves.