One of the basic principles of medieval medicine was that of the four humors of which a human body was composed. The balance between these four (blood, phlegm, yellow bile, and black bile) was essential for the well-being of a person.
Medieval Europe holds many of its foundations in the classical world of Ancient Greece and Rome. Just the same, "the theory of the four humours (bodily fluids) arose out of Hellenic philosophy in an attempt to relate all things to universal laws" (Cameron 159). And so we have parallels drawn between particular aspects of the surrounding world. The humors were oftentimes attributed to appropriate seasons, properties such as hot, cold, dry, and wet, signs of Zodiac in groups of three, four ages of mankind - infancy, adolescence, adulthood, and old age, and even sometimes to the four Evangelists - Matthew, Mark, Luke, and John, as well as the compass directions. (Even now, we still use words "choleric", "sanguine", "phlegmatic" and "melancholy" to describe personalities.)
Such balance and classification of the world was of utmost importance, hence the balance of humors within a human body was necessary for health. Balance of humors in humans was achieved by diet, medicines, and phlebotomy (blood-letting).
The eating habits of a medieval person depended mostly on the geographical location and financial status. Obviously, persons of low income (about 90% of the population in the feudal social system of medieval Europe) could not afford extravagant additions to their diets such as spices or sugar. In addition, transport of food was either outrageously expensive or just impossible due to the lack of a good method of preservation of food - obviously refrigeration has not yet been invented. As a result, a common medieval person's diet consisted of either wheat, meat, or fish, depending on location. Only two groups were combined together at most in many cases. This diet would be supplemented with the available vegetables, fruit, or herbs available during the season in the area.
This is not to say though that a medieval person did not realize the importance of a proper diet. Many sources cite the physicians' opinions on the significance of a well-balanced diet. In addition, many guiding writings recorded as conversations with medieval students exist to show that the medievals cared for appropriate nutrition. Most of the cases provide examples of diets somewhat parallel to the food pyramid created by modern science.
An example eating habit to pursue by a medieval person would be as follows: first meat, for it is most easily available, then vegetables, eggs, fish, cheese, butter, and beans, and eventually either water or ale, and in cases of exceptional wealth - wine.
For advice on proper lifestyle of a medieval person, please consult Aelfric's Colloquy.
The medicines in the Middle Ages more often than not would take the form of herbal remedies. In accordance with the humor theory, most plants, food substances, and commonly found house items were specified as either cold, hot, dry, or wet so that they could be used to modify the amounts of humors within a person. For example, pasta (warm) would be used for hot stomach, cold and dry linen could be applied to dry up ulcerations, and sugar (humid) produced "blood that [was] not that bad". (From Tacuinum Sanitatis - The Medieval Handbook).
For more information on actual healing methods, please see the healing section of this site.
Whenever medieval medicine is mentioned in a conversation among persons of our modern age, the first uttered words usually pertain to the practice of blood-letting in the Middle Ages. It is probably due to the shocking value of such treatment as well as its eventual failure and ineffectiveness in most cases.
Phlebotomy (blood-letting) was considered by medieval medicine to be a form of surgery. This view was based on the belief that each organ within a human body had its own organ of origin and, therefore, letting the blood from a specific vein would affect a particular organ. "It was not enough that a patient be bled, he must be bled from a proper vessel. There was a theory that various internal organs were connected with various superficial veins, so that bleeding from these veins drew noxious humours from organs which could not otherwise be reached" (Cameron 165). Because the internal organs were to be in a way worked on, phlebotomy became a surgical procedure.
Blood-letting allowed for the control of humors in a particular part of the body. Phlebotomy was administered in two ways, via derivation or revulsion. Derivation meant letting of blood at a point close to the affected area, and revulsion meant that blood was let at the most remote point to the affected area. Both methods had specific indication for use in the case of different illnesses and were widely employed by medieval physicians.
The dangers of blood-letting are obvious; infection, weakening of the already sick organism, cutting up an artery instead of a vein causing unstoppable bleeding, accidental cutting of nerves, and the loss of consciousness by the patient were the most common issues a medieval doctor had to deal with while administering phlebotomy. More often than not, the result of blood-letting was either continual sickness or death of a patient.
As improbable as it sounds, phlebotomy fit perfectly within the humor theory. "Blood withdrawn from a normal individual forms a homogeneous clot, but blood withdrawn from one suffering from any number of pathological conditions separates not layers as it clots, the red corpuscles sinking so fast that the clotted blood consists of a lower, red part, and an upper, yellow part" (Cameron 160). Such observation along with common symptoms such as the excretion of phlegm during a regular cold, or even "black bile" during the bowel movement were probably the reasons for the concoction of the faulty humor theory.
Additionally, a lot of medieval medicine had its roots in Greek practices. For example, one of the most famous Greek physicians, Galen (c. 130 - c. 200) had written a monumental treatise covering every aspect of medicine. In this work he addressed various issues (such as that food turns to blood in the liver, flows in vessels, and on reaching the heart, flows through pores in the septum from left to right side and is sent on its way by heart spasms), but "forgot" to include that of blood circulation. The idea of circulation of blood was not widely accepted and taught to physicians until well into the 16th century, when the medieval period ceased to exist.
Faulty observation and misdiagnoses built the foundation for the theory of humors as the major medical explanation for health disorders of the medieval peoples. For example, the most common time for phlebotomy was at the end of Lent (the 40 day period before Easter, during spring). After a long and cold winter, during which the major food group for most was salted meat, many persons have developed the first symptoms of scurvy out of the lack of vitamin C in their diet. As soon as Lent began, meat was prohibited according to the Catholic tradition, and the diet was soon implemented with other ingredients, especially fresh herbs and vegetables again available during the spring time. The improved diet obviously resulted in improved health of a medieval person. On the other hand, at the same time phlebotomy was administered in order to balance out the accrued "bad" humors of winter. The final result was that phlebotomy, rather than the improved diet, was perceived as the immediate cause of improved health status of the population.
More detailed information on the treatment of particular ailments during the Middle Ages can be found in Tacuinum Sanitatis (The Medieval Handbook) - a collection of colored plates assigning humoral qualities to everyday items.