The Black Death

Miasma Theory and the Causes of Plague

For the physicians, theologians, and scholars of the fourteenth century, the question of what caused the Black Death was not merely academic — it was existential. If the cause could be identified, perhaps it could be avoided or countered. If it could not, then the plague was an act of God or nature that humanity was powerless to resist. The dominant explanation — that the plague was caused by corrupted air, or miasma — shaped the medical, political, and personal responses to the pandemic in ways that had both beneficial and harmful consequences.

The Concept of Miasma

The word “miasma” derives from the Greek μίασμα, meaning “pollution” or “defilement.” In ancient and medieval medical thought, miasma referred to corrupted air — air that had been contaminated by poisonous vapours arising from decaying organic matter, stagnant water, volcanic activity, or other sources. When inhaled, this corrupted air entered the body, disturbed the balance of the humours, and caused disease.

The miasma theory was not a single, unified doctrine but a family of related ideas that had developed over centuries. Hippocrates, in his treatise Airs, Waters, Places, had argued that environmental factors — the quality of the air, the direction of the winds, the proximity of swamps and marshes — influenced the health of populations. Galen had elaborated on this idea, linking the corruption of the air to specific diseases and recommending avoidance of pestilential locations as a preventive measure.

By the fourteenth century, the miasma theory was the standard explanation for epidemic disease in both the Christian and Islamic worlds. The Arabic medical tradition, drawing on the same Hippocratic-Galenic heritage, used the term waba (pestilence) to describe corrupted air that caused epidemic disease, and the physicians Ibn al-Khatib and Ibn Khatima discussed the role of corrupted air in the transmission of plague alongside their observations of contagion.

The Paris Report of 1348

The most authoritative statement of the miasma theory as applied to the Black Death was the report produced by the Medical Faculty of the University of Paris in 1348. Commissioned by King Philip VI of France, the report was the product of the faculty’s collective deliberation and drew on the best available medical and astrological knowledge.

The report attributed the plague to a conjunction of three planets — Saturn, Jupiter, and Mars — in the zodiacal sign of Aquarius, which had occurred on 20 March 1345. According to the faculty’s reasoning, this celestial event had set in motion a chain of atmospheric disturbances. Saturn, which was cold and dry, had corrupted the air with its malevolent influence. Jupiter, which was hot and moist, had drawn up pestilential vapours from the earth and water. Mars, which was hot and dry, had ignited these vapours, producing the corrupted air that caused the plague.

The report recommended a series of preventive measures based on this analysis. People should avoid exposure to the south wind, which was thought to carry the most dangerous vapours. They should burn aromatic woods and herbs — juniper, rosemary, sage, laurel — to purify the air in their homes. They should avoid excessive exercise, bathing, and rich foods, which were thought to open the pores and allow corrupted air to enter the body. And they should flee from infected areas if possible.

The Paris report was widely circulated, translated into multiple languages, and cited by physicians and chroniclers across Europe. It remained the standard explanation for the plague’s cause for decades and influenced the medical response to subsequent outbreaks.

Practical Implications of Miasma Theory

The miasma theory had significant practical implications. If corrupted air caused disease, then measures to purify the air or avoid it were rational preventive strategies. The burning of aromatic substances — incense, juniper, rosemary, vinegar — was widely practised and may have had some beneficial effects: the smoke may have repelled fleas, and the strong odours may have masked the smell of decay that was associated with plague-stricken areas.

The recommendation to flee infected areas was the most effective preventive measure suggested by the miasma theory. Those who left plague-stricken cities for the countryside reduced their exposure to the rats and fleas that were the true vectors of the disease, even though they did not understand the mechanism. The practice of flight became so widespread among the wealthy and powerful that it was satirised by Boccaccio in the Decameron, whose frame narrative depicts a group of young Florentines who flee the plague-stricken city for a country villa.

The miasma theory also informed the design of public health measures. The cleaning of streets, the removal of waste and corpses, and the regulation of burial practices were all motivated by the desire to eliminate sources of corrupted air. These measures, while based on an incorrect theory of disease causation, had the incidental benefit of reducing the environmental conditions that favoured rat and flea populations.

Competing Explanations

The miasma theory was not the only explanation for the plague. Alternative theories attributed the disease to divine punishment, to poisoning by Jews or other minority groups, to the influence of comets or earthquakes, or to the corruption of water supplies. These theories coexisted with the miasma theory and were sometimes combined with it.

The theory of divine punishment was widely held, particularly among the clergy. If God had sent the plague as punishment for human sin, then the appropriate response was repentance, prayer, and penance — not medical treatment or flight. This view was not necessarily incompatible with the miasma theory: God could work through natural causes, sending corrupted air as the instrument of His wrath. But it placed limits on the perceived efficacy of medical intervention and contributed to the crisis of faith that followed the plague.

The theory of poisoning — particularly the accusation that Jews had poisoned wells to spread the plague — was widely believed in the Rhineland and other parts of Central Europe, despite the lack of evidence and the condemnation of Pope Clement VI. This theory had no basis in medical science, but it provided a scapegoat for a catastrophe that defied rational explanation and fed into existing antisemitic prejudices.

The Contagion Debate

One of the most significant intellectual debates of the plague era concerned the question of whether the disease was contagious — that is, whether it could be transmitted from person to person or from place to place. The miasma theory, in its purest form, held that disease was caused by local environmental conditions (corrupted air) and was not transmitted between individuals. If the air in a particular location was corrupt, then anyone who breathed it would become ill, regardless of whether they had been in contact with sick people.

However, empirical observation strongly suggested that the plague was contagious. The disease spread from city to city, from house to house, and from person to person in patterns that could not be explained by local environmental conditions alone. Physicians such as Guy de Chauliac in Avignon and Ibn al-Khatib in Granada observed that the plague was transmitted by contact with the sick, their clothing, or their belongings. Ibn al-Khatib cited the example of a single garment that had transmitted the disease to an entire household, and the example of isolated communities that remained free of plague until they came into contact with infected travellers.

The recognition of contagion had important practical implications. If the plague was contagious, then quarantine — the isolation of the sick and the restriction of contact with infected persons and goods — was a rational preventive measure. The Italian city-states, which implemented the earliest quarantine regulations, were influenced by the empirical observation of contagion, even though the theoretical framework to explain it was lacking.

The Decline of Miasma Theory

The miasma theory remained the dominant explanation for epidemic disease for centuries after the Black Death. It was not displaced until the nineteenth century, when the work of Louis Pasteur, Robert Koch, and other pioneers of bacteriology demonstrated that specific microorganisms caused specific diseases. The identification of Yersinia pestis as the cause of plague in 1894 was one of the triumphs of the new germ theory and provided the modern understanding that finally replaced the miasma framework.

However, the miasma theory was not simply wrong in every respect. Its emphasis on environmental factors — sanitation, ventilation, waste removal — led to public health measures that genuinely reduced disease transmission, even though the theoretical explanation was incorrect. The cleaning of streets, the regulation of burial practices, and the improvement of urban sanitation that were motivated by miasma theory all contributed to the reduction of rat and flea populations and the control of plague in subsequent centuries.