Germ theory
The idea, now proven, that many diseases are caused by microscopic organisms spreading between people, not by bad air.
The germ theory of disease holds that many illnesses are caused by microorganisms, such as bacteria, viruses and parasites, that invade the body and can pass from one person to another. It seems obvious now, but for much of the 19th century many doctors favoured the miasma theory: that epidemics came from foul, poisonous air rising from rotting matter.
The London physician John Snow argued in 1849 that cholera spread through water contaminated with sewage, carried by some unseen agent. During the Broad Street outbreak in Soho in 1854, which killed 420 people, he mapped the deaths and showed that they clustered around one public water pump; the parish authorities removed its handle. He also compared houses supplied by two water companies and found that those receiving sewage-polluted Thames water had a cholera death rate 14 times higher than those with cleaner water. Experiments by Louis Pasteur from the 1860s, and later work by Robert Koch and others, put germ theory on a firm footing.
The theory explains why clean water matters so much. Filtering drinking water slowly through sand, where a living film of microbes traps and consumes bacteria, can stop a waterborne disease at a city's boundary.
Not every widespread disease is caught, though. Some, such as scurvy and pellagra, come from missing nutrients, and early investigators sometimes mistook them for infections.
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