The Black Death

Medieval Medicine and Humoral Theory

The physicians who confronted the Black Death in 1348 were not ignorant or superstitious. They were the products of a sophisticated intellectual tradition that had developed over centuries, drawing on the accumulated learning of Greek, Roman, Arabic, and Latin medical authorities. Their framework — the theory of the four humours — was coherent, well-supported by authoritative texts, and capable of explaining most of the diseases they encountered in ordinary practice. It was, however, fundamentally inadequate to the challenge posed by a novel epidemic of unprecedented scale and lethality.

The Humoral Framework

The theory of the four humours originated in the medical writings attributed to Hippocrates of Cos (c. 460–370 BCE) and was systematised by the Roman physician Galen of Pergamon (129–c. 216 CE). According to this theory, the human body contained four principal fluids: blood, phlegm, yellow bile (choler), and black bile (melancholy). Each humour was associated with specific qualities — blood was hot and moist, phlegm was cold and moist, yellow bile was hot and dry, and black bile was cold and dry — and with specific organs, seasons, and temperaments.

Health was defined as the proper balance of these humours within the body. Disease resulted from their imbalance — an excess or deficiency of one or more humours, or a corruption of their quality. The physician’s task was to diagnose the nature of the imbalance and to restore equilibrium through appropriate interventions: bloodletting to reduce excess blood, purging to eliminate excess bile, dietary adjustments to modify the production of humours, and the administration of medicines designed to counteract specific humoral imbalances.

This framework was not merely theoretical. It was supported by centuries of clinical observation and scholarly commentary. The works of Galen, translated into Arabic in the eighth and ninth centuries and then into Latin in the twelfth and thirteenth, formed the core of the medical curriculum at the great European universities. The Canon of Medicine by Avicenna (Ibn Sina, 980–1037), which synthesised Greek medical knowledge with Arabic clinical observation, was the standard textbook at Montpellier, Bologna, Paris, and other leading medical schools.

Medical Education in the Fourteenth Century

The training of a medieval physician was rigorous and lengthy. Students typically began with the study of logic, natural philosophy, and astronomy before proceeding to medicine proper. The medical curriculum included the study of anatomy (primarily from texts, though dissection was increasingly practised at Italian universities), the theory of the humours, the properties of medicines (the materia medica), and the art of diagnosis through examination of the pulse, urine, and other signs.

The University of Paris, whose Medical Faculty produced the famous report on the causes of the plague in 1348, was one of the most prestigious medical schools in Europe. The faculty’s members were trained in the authoritative texts and were accustomed to reasoning from first principles — the qualities of the humours, the influence of the stars, the properties of the elements — to explain specific diseases. This approach, while intellectually rigorous, was ill-suited to a disease that did not conform to any previously observed pattern.

The distinction between physicians and surgeons was sharp. Physicians, who held university degrees, diagnosed diseases and prescribed treatments. Surgeons, who were trained through apprenticeship rather than university study, performed operations, set bones, and treated wounds. Below both were the barber-surgeons, who performed bloodletting and other minor procedures, and the apothecaries, who prepared and dispensed medicines. The plague, which required both diagnosis and physical intervention, exposed the limitations of this hierarchical division of labour.

Diagnosis and the Signs of Disease

Medieval physicians diagnosed disease through the careful observation of signs. The pulse was examined for its rate, strength, and regularity. The urine was inspected for its colour, clarity, sediment, and odour — a practice known as uroscopy, which was depicted in countless medical manuscripts and was one of the most characteristic activities of the medieval doctor. The patient’s complexion, breathing, sweat, and excretions were all examined for clues to the underlying humoral imbalance.

The symptoms of bubonic plague — the sudden onset of fever, the appearance of painful buboes in the groin, armpit, or neck, the rapid progression to death — did not fit neatly into the humoral framework. The buboes were sometimes interpreted as the body’s attempt to expel corrupt humours through the lymph nodes, and their appearance was seen as a sign that the disease was located in the specific humour associated with the affected region. However, the speed and lethality of the disease defied the gradual, balanced approach that humoral medicine favoured.

The symptoms of pneumonic plague — the coughing of blood, the rapid death within one to three days — were even more difficult to explain within the humoral framework. The spitting of blood suggested a corruption of the blood humour itself, which was considered the most vital and life-sustaining of the four fluids. When the blood was corrupted, the prognosis was almost invariably fatal, and physicians had few effective interventions.

The Role of Astrology

Medieval medicine was closely linked to astrology. The positions of the planets and stars were believed to influence the body’s humours and to determine the susceptibility of individuals and populations to disease. Medical astrology assigned each planet and zodiacal sign to specific body parts and humours, and physicians consulted astrological tables when planning treatments, scheduling bloodletting, and predicting the course of diseases.

The Medical Faculty of Paris, in their famous report of 1348, attributed the plague to a conjunction of Saturn, Jupiter, and Mars in the sign of Aquarius. Saturn, associated with cold and dryness, was thought to corrupt the air; Jupiter, associated with heat and moisture, was thought to draw corrupt vapours from the earth; and Mars, associated with heat and dryness, was thought to ignite these vapours and produce pestilence. This explanation, while seemingly fantastical to modern readers, was consistent with the intellectual framework of the time and was accepted by many learned physicians.

The astrological explanation had practical implications. If the plague was caused by celestial influences, then it was beyond human control — a conclusion that reinforced the sense of helplessness that characterised the medical response. However, the astrological framework also suggested that certain times and places were more dangerous than others, and physicians used astrological calculations to advise patients on when to flee, when to take medicines, and when to avoid bloodletting.

The Limits of Humoral Medicine

The fundamental limitation of humoral medicine in the face of the Black Death was its inability to account for contagion — the transmission of disease from one person to another. The humoral framework explained disease as an internal imbalance, not as an external agent that could be transmitted between individuals. While some physicians, notably Guy de Chauliac and Ibn al-Khatib, observed that the plague appeared to spread from person to person and from place to place, this observation could not be easily reconciled with the humoral model.

The concept of semina morbi (seeds of disease) — the idea that disease could be transmitted by tiny particles or “seeds” that passed from one body to another — had been proposed by the Roman scholar Marcus Terentius Varro in the first century BCE and was occasionally discussed in medieval medical literature. However, this concept was not widely accepted and was not integrated into the mainstream humoral framework. The full development of germ theory would not occur until the nineteenth century, with the work of Louis Pasteur and Robert Koch.

The Legacy of Medieval Medicine

The encounter with the Black Death did not destroy the humoral tradition. The framework remained dominant in European medicine for another three centuries, until it was gradually displaced by the mechanical philosophy of the seventeenth century and the germ theory of the nineteenth. However, the plague did stimulate important developments within the tradition. The recognition that epidemic disease required new approaches led to increased emphasis on observation, the collection of case histories, and the development of specialised plague treatises that drew on direct experience rather than solely on authoritative texts.

The quarantine measures developed by the Italian city-states represented the most significant innovation of the plague era. These measures, which were based on the empirical observation that isolation could prevent the spread of disease, did not require a theoretical explanation of the disease’s cause. They worked regardless of whether the plague was caused by corrupted air, planetary conjunctions, or invisible seeds of disease. In this sense, the practical response to the plague anticipated the theoretical understanding by several centuries.