The Broad Street pump, and John Snow's development of epidemiology.
Thanks to his mother's support, John Snow rose from humble beginnings as a coal miner's son and apprenticed to a doctor in Newcastle. As a young man, he treated many patients during the cholera epidemic that struck Newcastle. He noticed that the traditional explanation for cholera's spread - miasma from graveyards and swamps - could not explain its appearance in Newcastle where he treated patients. He took that knowledge with him to London, where he formally studied medicine and achieved the highest honors in his profession in only a year. His formal study of anesthesia earned him such great recognition that on two occasions he was trusted to work on the Queen. But then cholera broke out in London again. Snow wanted to prove miasma didn't cause it and find the real cause, so he interviewed patients and doctors across the city. He theorized that the diarrhea which came from cholera also helped to spread it. He even wrote up a case study where one street whose well water mixed with sewage had a huge infection rate while across the street their neighbors with pure well water barely suffered at all. Confident that he had found the cause, he published his findings, but the medical community was not thoroughly convinced.
John Snow's single case study was not enough to convince the medical community that cholera was spread through the water, but he did not give up. He founded the Epidemiological Society of London in 1850, the first organization dedicated to studying not just cures for disease, but also their causes. And so when cholera returned in 1854, John Snow saw a chance to finally prove his theory and set about studying the patterns of disease. The disease appeared to strike randomly, both rich and poor, but he realized that in his district were two different water companies, one of which he theorized might be contaminated. Finding evidence proved more difficult than he anticipated: going to door to door, he was often met by people who didn't even know what water company supplied their building. He tracked down landlords and even developed a water test to help him identify which water source each house had, but before he had the time to compile and analyze his findings, another terrible outbreak struck in Broad Street.
John Snow raced to discover the causes of the cholera epidemic that swept Broad Street. He went door-to-door talking to the locals, then surveyed government records for extra clues. He began to craft a map of deaths, and drew the first Voronoi Diagram to assess the victims' proximity to the pump. All but 8 of the 84 victims were closer to the Broad Street Pump (and hence more likely to use it) than any other pump, and most of the remainder had daily commutes that took them past the pump. He also noticed that a local workhouse and a tavern were conspicuously cholera-free despite their proximity to the pump, and found that they had access to their own drinking supplies which unbeknownst to them had kept them safe. With his evidence in hand, he met with the local health commission and convinced them to deactivate the Broad Street Pump. But his theory was still not widely accepted, and after the epidemic passed everything returned to normal. At last, a local pastor named Henry Whitehead set out to debunk the wild theories about what had caused the epidemic in his parish. He doubted Snow's results, but as he investigated, he found more evidence that backed them up. His relationship with the neighborhood also meant he could get information Snow couldn't, and it was thus that he found Patient Zero: a baby who died two days before the epidemic, and whose mother had been throwing her dirty diapers in a cesspit under the house. The government investigated and found that the poorly-built cesspit had begun leaking into the Broad Street Pump's water supply, infecting all who drank from it with cholera passed along in the baby's diapers. It would take many years before John Snow's theory became accepted fact, but his research paved the way for the modern medical field of epidemiology.
John Snow's report on the causes of cholera provided yet more evidence of the dangers of filthy cities. Cities had always been unhealthy places to live, generally with a higher death rate than birth rate, but fixing them just wasn't the focus of an agricultural world economy. The Industrial Revolution in the 1700s brought more people to the cities, and suddenly, cities had to grow in order to maintain the vastly expanded manufacturing and shipping operations of the new era. Edwin Chadwick published a report about the sewage in city streets and clearly explaining the need to remove it. His report led to legislation that created local health boards and drove the construction of complex sewer systems. These sewers were massive, expensive undertakings that, even today, remain the foundation of many large modern cities. They reduced diseases across the board and saved countless human lives, part of a legacy that John Snow would be proud of.