The underlying principle of medieval medicine was the theory of humours. This was derived from the ancient medical works, and dominated all western medicine up until the 19th century. The theory stated that within every individual there were four humours, or principal fluids – black bile, yellow bile, phlegm, and blood, these were produced by various organs in the body, and they had to be in balance for a person to remain healthy. Too much phlegm in the body, for example, caused lung problems; and the body tried to cough up the phlegm to restore a balance. The balance of humours in humans could be achieved by diet, medicines, and by blood-letting, using leeches. The four humours were also associated with the four seasons, black bile-autumn, yellow bile-summer, phlegm-winter and blood-spring.

The astrological signs of the zodiac were also thought to be associated with certain humours. Even now, some still use words “choleric”, “sanguine”, “phlegmatic” and “melancholy” to describe personalities.
The use of herbs dovetailed naturally with this system, the success of herbal remedies being ascribed to their action upon the humours within the body. The use of herbs also drew upon the medieval Christian doctrine of signatures which stated that God had provided some form of alleviation for every ill, and that these things, be they animal, vegetable or mineral, carried a mark or a signature upon them that gave an indication of their usefulness. For example, the seeds of skullcap (used as a headache remedy) can appear to look like miniature skulls; and the white spotted leaves of Lungwort (used for tuberculosis) bear a similarity to the lungs of a diseased patient. A large number of such resemblances are believed to exist.
Most monasteries developed herb gardens for use in the production of herbal cures, and these remained a part of folk medicine, as well as being used by some professional physicians. Books of herbal remedies were produced, one of the most famous being the Welsh, Red Book of Hergest, dating from around 1400.

medieval medicine summery. The majority of that is not relevant to today’s medicine. That is all I am saying