After the Romans left Britain, medicine changed a lot. The Romans took their medical knowledge with them and the British had to start from scratch. People forgot the importance of keeping clean and mostly blamed their sins for illnesses. Everybody believed strongly in God and that their faith would keep them healthy. After a few centuries, hospitals and universities started to appear, run by the Church, which was the richest and most powerful organization in Britain at the time. They spread the teachings of Galen and the Theory of Humours, and these remained popular for centuries. Muslim scholars had translated all Galen's books into Arabic so they weren't lost forever with the Romans. Some things were, though - like bath houses and flush toilets. Medieval Britain must have reeked!