Leeches are annelids, a kind of worm (ground creature) that lives in wet places. All leeches are carnivorous, that is, they eat mainly meat, but some are also haemophagic; this means they drink the blood of other animals. They are sometimes helpful in medicine. They are often used to reduce blood clots. They were used to suck the blood of patients a long time ago, before we knew about germs and that losing blood is bad for health.