Cholera is a deadly disease that causes severe diarrhea and vomiting, leading to dehydration and death within hours if left untreated. It is caused by a bacterium called Vibrio cholerae, which infects the intestines and produces a toxin that damages the cells lining the gut. Cholera is usually spread through contaminated water or food, especially in areas with poor sanitation and hygiene.
But in the mid-19th century, most people did not know how cholera was transmitted. They believed that it was caused by breathing bad air, or “miasma”, which arose from decomposing matter or other dirty organic sources. This theory was widely accepted by the medical community and the public, even though it could not explain why some people got sick and others did not, or why cholera outbreaks occurred in certain places and not others.
One of the people who challenged the miasma theory was John Snow, a physician and a pioneer of anaesthesia and medical hygiene. He suspected that cholera was spread through tainted water supplies, not bad air. He had observed that cholera cases were more common among people who drank water from the Thames River, which was polluted with sewage and industrial waste. He also noticed that people who boiled their water or drank other beverages, such as tea or beer, were less likely to contract the disease.
To test his hypothesis, Snow conducted a series of investigations during the cholera epidemics that plagued London in 1848 and 1854. He collected data on the location, timing and severity of cholera cases, as well as the source of water for each household. He plotted this information on maps, which revealed patterns and clusters of infection. He also interviewed the survivors and relatives of the victims, asking them about their water consumption habits and preferences.
One of his most famous maps showed the distribution of cholera cases around a water pump on Broad Street (now Broadwick Street) in Soho, London, during an outbreak in 1854 that killed 616 people. Snow found that nearly all the deaths had occurred within a short distance of the pump, and that most of the victims had drunk water from it. He also found exceptions that supported his theory: for example, a brewery nearby had no cholera deaths because its workers drank beer instead of water; a prison in the same area had no cases because it had its own well; and some people who lived far away from the pump but liked its water had also died.
Snow concluded that the Broad Street pump was contaminated with cholera and that removing its handle would stop the outbreak. He presented his findings to the local authorities, who were initially skeptical but agreed to his request. The pump was disabled on September 8th, 1854, and the epidemic soon subsided.
Snow wrote in his report: “There is no doubt that the mortality was much diminished, as I said before, by the flight of the population, which commenced soon after the outbreak; but the attacks had so far diminished before the use of the water was stopped, that it is impossible to decide whether the well still contained the cholera poison in an active state, or whether, from some cause, the water had become free from it.”
Snow’s work was groundbreaking for several reasons: he used scientific methods to collect and analyze data; he used maps as a visual tool to communicate his results; he conducted a natural experiment by comparing two groups of people with different water sources; and he proposed a practical solution to prevent further infections.
However, Snow’s discovery was not widely accepted or appreciated at the time. He faced ridicule and opposition from his peers and officials who clung to the miasma theory. His work was only recognized decades later, after germ theory was established by Louis Pasteur, Robert Koch and Joseph Lister. Snow is now considered one of the founders of modern epidemiology, the study of diseases and their causes.
One of Snow’s admirers was Florence Nightingale , who wrote in 1865: “The late Dr John Snow has been much abused for his views on cholera; but I believe he will be right at last.”
Snow’s story shows how one person can make a difference by challenging conventional wisdom and using evidence to support his claims. His map of Broad Street is an iconic symbol of scientific inquiry and public health intervention. His legacy lives on in the field of epidemiology, which continues to use maps and data to track and control diseases around the world.