For a brief rundown of the time periods discussed in this answer, please refer to this post on the breakdown of medieval historical ages. This is particularly important because medical theory and treatment differed significantly in the Late Middle Ages compared to Late Antiquity and Early Middle Ages. Both draw on the teachings of Galen (129 AD - 200 AD), a classical physician responsible for spread of humoral theory; however, widespread practice and teaching of humoral theory in the Middle Ages was not revived in Europe until after the twelfth century.
What is humoral theory?
Humoral theory, or the belief that the body is divided into four humors which determine temperament and health, was the cornerstone of medieval medical practice. In the Late Middle Ages, physicians employed humoral theory in their treatment of diseases. Diseases and personality traits were explained through an imbalance or concentration of one humor in favor of another.
Common treatments using humoral theory include the application of heat (to treat an overbalance of Phlegm or Black bile) or bloodletting (to counteract a disease of the liver/over concentration of Blood). Sometimes, physicians would prescribe specific diets to counteract the disease. Someone who was sweating and feverish (wet and hot) would be made to eat cold and dry foods.
Crucial to the proliferation of medieval texts on humoral theory was The Canon of Medicine, an eleventh-century encyclopedia of medicine by the Persian philosopher Avicenna. In the Early Middle Ages, in the wake of the fall of Rome and widespread decentralization, most of the medical texts were either lost or not copied. The years after the First Crusade witnessed the flood of scientific and medical literature into Europe from Islamic lands, where the Greco-Roman medical tradition had been preserved. Avicenna’s Canon was essential in reintroducing humoral theory to Europe. With the knowledge from Arabic texts and those precious codices that had survived in monasteries in Europe, the Schola Medica Salernitana in Salerno, Italy was founded. It was Europe’s first medical school, and it taught humoral theory.
For more information on humoral theory in treating diseases and diet, see the links below:
[Note: these links concern either Late Medieval Medicine or humoral theory in the Classical world.]
Medieval medical practitioners employed a variety of herbal techniques in the treatment of diseases and wounds. The most common cures included herb-infused teas, poultices (for rashes or wounds), ointments, and salves. Often, heat was used in conjunction with the herbal treatment. For example, to dispel a fever, the doctor or midwife might wrap the patient in blankets and light a fire in the room.
When treating wounds from an arrow or sword, doctors understood that loss of too much blood could lead to death. As part of their training, knights and squires gained a basic knowledge of treating wounds on the battlefield, including wrapping the wound in cloth (although, they had no knowledge of germ theory, so the cloths were not necessarily clean). Off the battlefield, the injured would be treated by one of the professionals mentioned below. In addition to herbal remedies, a physician might cauterize the wound to prevent further bleeding.
Throughout human history, herbs have been the basis for most medicine. Medieval doctors, nurses, and midwives made use of herbs, drawing on knowledge preserved either in books called Herbals or through oral tradition. The medieval recipes that have survived contain both herbs and animal products, such as ground bones, milk, or fat. Honey and chamomile are two plant products that are often repeated throughout medieval recipes. This link provides a list of herbal associations. Also, check out the Medieval Herb List.
Many of the recipes that have survived are often paired in the same manuscript as devotional material. At the same time that a poultice was applied, the medical practitioner and the sick would recite a prayer to God. Disease was ultimately caused by the will of God. Medieval people often prayed or gave confession to ensure their recovery. Health and piety were clearly linked in the medieval mindset. One author, Henry of Lancaster, composed his own book of medicine to cure the soul, Le Livre de seyntz medicines (The Book of Holy Medicines). In it, Henry links curing a headache to curing oneself of sin, drawing on imagery of the Virgin Mary in his recipe for an herbal cure. Part of this culture included going on pilgrimage to purge oneself of disease.