The Middle Ages was a grim time to be poorly. In the 1350s, the average life expectancy was perhaps 30-35. Infant mortality was extremely high where 1 in 5 children died before their first birthday and many women died in childbirth.
People died from simple injuries, diseases such as leprosy (a disease affecting parts of the body and the nervous system) and smallpox (a viral disease with fever and sores) and various fevers. Nearly a thousand years after the fall of Rome, medicine in Europe had regressed and returned to a more primitive outlook. Treatments continued to be a mixture of herbal remedies, bleeding and purging, and supernatural ideas.
Medieval doctors did not have a clue what caused disease.
Most doctors still believed the Greek theory from Galen, a doctor during the Roman Empire, that you became ill when the 'Four Humours' - phlegm, black bile, yellow bile, blood - became unbalanced. During the medieval era dissection of human bodies was banned so doctors didn’t properly understand what went on inside the body.
But they also blamed everything from the stars, to demons, to sin, to bad smells. They trusted supernatural ideas that included God, charms and luck, witchcraft or astrology.
During epidemics, people would blame witches, nobility or groups who were culturally different such as Jewish people, and attack them. When a disease like the Black Death hit England in 1348, the doctors were powerless to stop it killing half the population. There were both supernatural and natural explanations for it, for example, some people said that God had sent it as a punishment, others that the planets were in the wrong conjunction, or that it was caused by 'foul air'.
Recently, historians have suggested that many medieval treatments were successful, especially the herbal remedies. Nevertheless, successful cures were mixed up with, what seem to us, crazy cures. For example:
bleeding, applying leeches, smelling strong posies or causing purging or vomiting
cutting open buboes, draining the pus and making the patient hot or cold, eg by taking hot baths
trepanning - cutting a hole in the skull
praying, or whipping themselves to try to earn God's forgiveness
lighting fires in rooms and spreading the smoke, tidying rubbish from the streets and banning new visitors to towns and villages