The Medieval Age ran from 500AD until 1500 AD. After the Roman Empire collapsed there was widespread chaos. Many ideas and technology was lost. The first part of this period is known as the Dark Ages, as much of what the Romans had learned and built was lost. There was disorganisation and many peoples and tribes fought each other instead of working together as they had under the Romans. This made travel and trade dangerous which slowed communication and the spread of ideas almost to a standstill.
The Catholic Church was the only thing to survive the collapse of the Empire. Slowly the Church took over as the powerful organisation in Europe. It used Christianity to continue to spread ideas about medicine as well as belief. This returned to a belief that God was the cause and cure of disease. The Church controlled ideas and only Monks could read, teaching took place in Monasteries which were also the only Hospitals. Natural ideas were discouraged, and dissection were forbidden. To go against the teaching of the Church brought heavy and harsh punishment. The church also controlled books, it supported Galen as he showed that a single God was important in treating the sick.
The advances that the Greeks and Egyptians had made were lost to medical knowledge. Only approved ideas were spread in the handwritten books that Monks copied out. They did not add their own knowledge, they just copied. They did write about the importance of some herbs and medicine, but mainly wrote about religious belief. Because these books were incredibly expensive and slow to finish the spread of ideas was slow.