Causes of illness and disease over time
Understanding Disease: A Changing Picture
For most of human history, people had no way of observing the microscopic world. Without the ability to see bacteria or viruses, explanations for disease had to come from observation, tradition and belief. Over 1,500 years, ideas about the causes of illness shifted dramatically — from supernatural forces and bad air to the germ theory that underpins modern medicine.
Medieval Ideas: Miasma, God and the Humours
In medieval Britain (c.500–1500), illness was explained in three main ways. First, many believed that miasma — foul-smelling air produced by decay — entered the body and caused sickness. Towns with open sewers and rotting rubbish were thought to be particularly dangerous, not because of germs but because of the smell itself.
Second, disease was widely seen as divine punishment. If a community sinned, God might send plague or pestilence as a warning or judgement. This belief meant that prayer, fasting and confession were considered as important as any physical remedy. Third, the theory of the four humours explained illness as an imbalance in the body’s fluids. Physicians tried to restore balance through bleeding, purging and adjusting diet.
Medieval people explained illness through miasma (bad air), divine punishment (God’s will) and the theory of the four humours (imbalance of bodily fluids). None of these explanations involved germs or infection in the modern sense.
The Black Death, 1348
The Black Death arrived in England in 1348, killing approximately one third of the population within three years. People had no way of knowing that the disease was spread by fleas carried on rats, or that it was caused by the bacterium Yersinia pestis. Instead, they blamed miasma, divine punishment, or the position of the planets. The responses this produced — burning herbs, flagellation, quarantine of towns — were based entirely on these mistaken beliefs. Quarantine, however, did accidentally help slow the spread in some communities.
The Great Plague of 1665 was the last major outbreak of bubonic plague in England, killing around 100,000 people in London alone — roughly a quarter of the city’s population. Despite being three centuries after the Black Death, the explanations offered were still largely the same: miasma and divine punishment. Physicians wore beak-shaped masks filled with aromatic herbs, believing this would filter out the bad air. The plague orders issued by the authorities ordered the killing of cats and dogs (which may have made things worse by removing some natural rat predators) and the lighting of fires in streets to ‘purify’ the air. The underlying belief — that foul smells caused disease — had barely changed.
Industrialisation and New Disease Challenges
The Industrial Revolution of the late eighteenth and nineteenth centuries created new threats to health. Rapid urban growth meant that thousands of people were crowded into slums with inadequate sewage systems and contaminated water supplies. Diseases such as cholera and typhoid thrived in these conditions.
Cholera first struck Britain in 1831–1832 and returned in 1848–1849 and 1853–1854. Typhoid was endemic in Victorian towns. Many educated people — including leading doctors — still blamed miasma. The smell from sewage and the Thames was blamed for the epidemics. This was wrong in its cause but accidentally correct in its consequence: cleaning up smells did sometimes mean cleaning up water, which genuinely stopped disease.
Industrialisation created overcrowded towns with poor sanitation, contaminated water and inadequate sewers. Cholera epidemics struck in 1831, 1848 and 1853. These conditions made urban disease far more deadly than in rural medieval England.
The decisive shift came when Louis Pasteur published his germ theory in 1861, demonstrating that microorganisms caused fermentation and decay. Robert Koch built on this between the 1870s and 1890s, identifying specific bacteria as the causes of specific diseases — including anthrax (1876), tuberculosis (1882) and cholera (1883). This was a revolution: for the first time, a scientific mechanism for how disease spread had been identified. Miasma was no longer just insufficient — it was demonstrably wrong.
Twentieth-Century Understanding: Bacteria, Viruses and Beyond
By the early twentieth century, scientists understood that diseases were caused either by bacteria (such as tuberculosis or cholera) or by viruses (such as influenza or smallpox). The 1918 influenza pandemic killed an estimated 50 to 100 million people worldwide — more than the First World War — demonstrating that even with improved knowledge, disease could still be catastrophic.
Later in the century, new diseases challenged the medical world. HIV/AIDS was first identified in 1981 and spread globally within years, caused by a retrovirus transmitted through blood and bodily fluids. COVID-19, caused by the SARS-CoV-2 coronavirus, emerged in late 2019 and became a global pandemic in 2020. Both diseases showed that new pathogens can emerge and spread rapidly even when medical science is highly advanced. Modern epidemiology tracks and analyses disease patterns across populations, enabling faster responses than were ever possible in medieval or even Victorian times.