If you live in the developed world, you’ve probably never had to worry about Cholera. It’s a preventable disease , and the way to prevent it is through proper sanitation. Unfortunately, that means that in many parts of the world, particularly peri-urban areas such as slums, outbreaks can still happen. Millions are affected annually, with tens of thousands of deaths. Still, even during an outbreak, many deaths can be prevented by effective treatment.
In 1854, the situation around Cholera was entirely different. It was a worldwide pandemic — in Russia alone, between 1847 and 1851, it killed over a million people . It affected all corners of the world, travelling between continents via land and sea.
The most accepted theory of how people got the disease was Miasma theory. The theory was that Cholera was caused by particles in the air, which came from things like decaying organic matter . As such, Miasma theorists did not believe that the disease was spread from person to person, meaning that the disease was not quarantined very effectively. Later, Louis Pasteur would propose Germ theory, but the idea that Cholera was spread by germs, and thus from person to person, was not yet accepted.
One skeptic of Miasma theory was a man named John Snow. Snow was an accomplished physician: he was a pioneer of Anesthesia, and even personally administered Chloroform to Queen Victoria during two of her child births. Snow was used to seeing Cholera: it surrounded him growing up, and he treated his first Cholera patients aged 19 as an apprentice surgeon . In 1837 he moved to London, where he would spend the rest of his career.
London was no exception to the Cholera pandemic. There was a sharp growth in population, as people migrated to the city in large numbers. As a result, many areas were crowded, and had poor sanitation. London did have a sewer system, but the city was growing too fast for new sewers to be built in time to serve the newest urban centres .
As a skeptic of Miasma theory, John Snow wondered what the real cause of Cholera might be. His experience of the disease led him to hypothesise that the disease was instead spread through contaminated water. This was not a commonly held belief by the physicians, or the authorities, at the time: he needed evidence if he was going to prove his theory, and get people to take effective sanitary measures.
One overcrowded area of London, with poor sanitation, was Soho: two previous outbreaks in 1832 and 1849 had killed a total of 14,137 people  — now, in 1854, another outbreak had occurred. Snow decided to investigate his theory here. He interviewed local residents, and mapped cases of the disease in the area. He published a map, shown below, of the incidence of Cholera (represented by stacked rectangles).
On the map are several pumps — these are where residents would go to collect their drinking water. Looking at the map, it is clear that the outbreak was centred around one pump in particular: the Broad Street Pump. This evidence was compelling enough to get the local parish to disable the pump by removing the handle, and the number of deaths in the area fell sharply.
This was a victory, and a breakthrough — but John Snow was just getting started. He was still not convinced — deaths were on the decline in that area anyway, and he hadn’t been able to detect Cholera in the water from the pump. As such, Snow wanted more proof. Here, he had a stroke of genius — it had been observed that different water companies in London had vastly different standards. Some, such as the Southwark and Vauxhall Company and the Lambeth Water Company, had been found to have animal hair (or worse) in their water. Other companies, such as the New River Company and Chelsea Company, filtered their water much more effectively, as well as drawing water from places other than the Thames, which in the absence of total sewer coverage, became a dumping ground for waste. Snow realised that this created perfect conditions for an experiment:
“In many cases a single house has a supply different from that on either side. Each company supplies both rich and poor, both large houses and small; there is no difference in the condition or occupation of the persons receiving the water of the different companies...As there is no difference whatever either in the houses or the people receiving the supply of the two Water Companies, or in any of the physical conditions with which they are surrounded, it is obvious that no experiment could have been devised which would more thoroughly test the effect of water supply on the progress of Cholera than this, which circumstances placed ready made before the observer. The experiment too, was on the grandest scale. No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentlefolks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge; one group being supplied water containing the sewage of London, and amongst it, whatever might have come from the cholera patients, the other group having water quite free from such impurity.” 
He meticulously collected figures on which houses were supplied by which water company, and how badly cholera had affected them. This provided much more evidence that there was a link between Cholera and contaminated water. Snow further proved this theory by observing prisons, and how once they changed their water sources, Cholera disappeared in a matter of days .
Despite the overwhelming evidence, no immediate action was taken by the government. It was felt that, because Snow claimed that Cholera could be transmitted through fecal matter, this theory was too disgusting to be considered .
Snow’s theories were finally accepted when a man named William Farr took his evidence into consideration. Farr had been an opponent of Snow and his theory, himself gathering data on how Cholera was linked to elevation levels, and suggesting that Cholera was influenced by many different factors, and was caused by Miasma. When Farr was investigating another outbreak, he realised the validity of Snow’s theory, and immediately ordered that water should not be drunk unless boiled . Afterwards, he collected further data linking Cholera to contaminated water, and this evidence was finally accepted as conclusive, leading to the improvements necessary to prevent the disease.
Snow’s work was key in discovering how to effectively prevent Cholera, and because of him, countless deaths have been prevented. Snow’s legacy is still seen in the fight against Cholera today: AI is being used to predict future Cholera outbreaks in developing countries, taking into account environmental factors and historical data. By being more like John Snow, perhaps we might one day reach a point where nobody has to die of Cholera.
 WHO Cholera Fact Sheet
 Geoffrey A. Hosking (2001). "Russia and the Russians: a history". Harvard University Press. p. 9. ISBN 0-674-00473-6
 UCLA Department of Epidemiology, Fielding School of Public Health: Competing Theories of Cholera
 Ball, Laura (2009). "Cholera and the Pump on Broad Street: The Life and Legacy of John Snow".
 UCLA Department of Epidemiology, Fielding School of Public Health: the Broad Street Pump
 Atlas Obscura: The Broad Street Pump
 Archive.org: John Snow's book, On the Mode of Communication of Cholera
 Frank Chapelle, Wellsprings, New Brunswick, New Jersey: Rutgers University Press, 2005, p. 82
 Cadbury, Deborah (2003). Seven Wonders of the Industrial World. London and New York: Fourth Estate. pp. 189–192.