Ideas About the Cause of Disease

To cure or prevent illness, people primarily sought spiritual forgiveness. A drastic, extreme method was flagellation, where individuals whipped themselves in public to demonstrate physical repentance and beg God to withdraw the plague.

Supernatural Explanations: Astrology

The position of the stars was once considered a matter of life and death. Alongside religion, medieval people turned to astrology—the study of planetary movements—to explain health issues. Physicians heavily relied on an almanac (a portable calendar of planetary positions) and star charts to diagnose their patients based on birth dates and the exact time they fell ill.

A crucial diagnostic tool was the Zodiac Man, a diagram connecting the twelve astrological signs to specific body parts. For example, Pisces governed the feet, while Aries controlled the head. Astrology dictated the safest times for medical procedures; a physician would refuse to perform bloodletting on a patient's arms if the moon was in Gemini, as it was considered malevolent for that region of the body.

Large-scale astrological events were frequently blamed for epidemics. Medical scholars at the University of Paris officially declared that a rare alignment of Saturn, Jupiter, and Mars in 1345 corrupted the atmosphere, causing the Black Death. Although initially suspicious of astrology as a form of fortune-telling, the Church eventually accepted it by teaching that God controlled the planets, making them a tool of His divine will.

Rational Explanations: The Four Humours

Imagine treating a winter cold by eating burning hot peppers. The most dominant rational explanation for disease was the Theory of the Four Humours, originally created by the Ancient Greek physician Hippocrates. He believed the human body was made up of four distinct liquids: blood, phlegm, yellow bile (choler), and black bile. According to this theory, a person remained perfectly healthy as long as these humours were perfectly balanced.

Illness occurred when the humours became unbalanced through an excess or a deficiency. Hippocrates pioneered clinical observation, a highly structured four-step process of observing symptoms, recording them, diagnosing the issue, and predicting the outcome.

The Ancient Roman physician Galen later developed this foundation into the Theory of Opposites. He argued that a humoural imbalance should be treated with a remedy possessing opposite qualities. For instance, since phlegm was linked to winter and considered "cold and wet," a cold (diagnosed as excess phlegm) would be rationally treated with "hot and dry" remedies like ginger, peppers, or wine.

Rational Explanations: Miasma Theory

If a street smelled like rotting garbage, medieval people believed the scent itself could kill you. The Miasma theory was the belief that disease was transmitted by noxious fumes or "bad air." People thought that inhaling foul-smelling, corrupted air from decaying organic matter, unwashed bodies, or stagnant water would immediately disrupt the body's internal humours and cause sickness.

During the Black Death, this theory was so prevalent that some people theorised earthquakes had cracked the ground open, releasing poisonous ancient air from deep within the earth to infect the population.

To protect themselves from miasma, people constantly tried to keep the air smelling sweet. Wealthy individuals carried a pomander, a small locket or pouch filled with fragrant herbs like lavender and rose, to shield their noses from the bad air. Towns also lit bonfires or rang church bells in an attempt to keep the air moving and disperse the noxious fumes.

Continuity: The Influence of the Church and Galen

Why did medical knowledge stay completely unchanged for over a thousand years? During the medieval period, there was a vast amount of continuity in medicine because the Roman Catholic Church tightly controlled almost all medical education. Monks and priests were the main literate groups responsible for hand-copying medical books, meaning the Church single-handedly decided which ideas were preserved and which were destroyed.

The Church aggressively promoted Galen's medical texts because his ideas fit perfectly with Christian theology. Even though Galen lived long before Christianity dominated Europe, he was monotheistic and wrote that the human body was perfectly designed by a single "Creator." Disagreeing with Galen was therefore treated as heresy and a direct challenge to the Church itself.

Because physician training at universities relied entirely on "book learning" rather than practical experience, Galen's many mistakes went unquestioned. His anatomical knowledge came from animal dissections (pigs and apes), leading him to incorrectly state that the human jaw had two bones and that the liver produced blood to be burned as fuel. The Church largely banned human dissection, insisting the body must remain intact for the afterlife. If a rare dissection ever revealed a contradiction to Galen's texts, physicians simply claimed the body belonged to a malformed criminal or that human anatomy had changed over the centuries.