John Snow and the cholera outbreak of 1854: Revealing an unintuitive truth using data

The map of cholera in Soho created by John Snow

Truth reveals itself to us in many different ways. Sometimes, it takes the form of an amazing revelation, an eye-catching explosion of color, or a terrifying act of nature. Other times, it takes on a more subtle form, discovered only through a combination of patience, knowledge, and determination.

Turn the clock back a few hundred years, and you would find a culture that did not have the sophisticated data analysis techniques to uncover the truths of natural world that we have today.  Claims were often backed up by “common sense”.  Society lacked a way of quantifying information and letting the data speak for itself. But this mentality began to change in the 1800s, marking an important shift in our scientific culture that continues to this day.  While the process spanned several generations and countless individuals, one of the more interesting stories is that of a man named John Snow.

The year was 1854, and London’s Soho district found itself in the midst of a horrific cholera outbreak.  The first case of the disease was documented on August 31st, and within a week 127 people had died. Nearly 75 percent of the residents had fled the area.  Authorities and leading medical practitioners were at a loss of ideas for how to solve this problem. Cholera had been a persistent problem in London, but it was unclear how it was transmitted from one person to another.  The leading theory at the time was known as “miasma” and suggested that cholera was transmitted via “bad air” that rested within the general area of the outbreak.

As a long-time skeptic of the “miasma” theory of disease transmission, John Snow — then a 41-year-old physician — wanted a more complete explanation for why some people get sick and others do not.  Germ theory was still several years away, so many of the explanations we would give today weren’t even considered back then.

In order to determine the true cause, Snow collected data on who was sick and where they lived.  He found that the largest number of cholera cases were concentrated around the Broad Street water pump. Pumps of that time were vastly different from today’s technologically advanced models. His finding suggested that contaminated water, not “bad air”, was responsible for outbreaks. He quickly made an appeal to the British government, and his data-based argument was convincing enough that they decided to disable the pump.  Within a week, new cases of cholera had plummeted. Afterwards, the government carried out an investigation of the area around the water pump and (unsurprisingly) found that it was situated next to an old cesspool, all but confirming that the disease had traveled through the water supply.

Snow created what is known as a “Veronoi Diagram” to split the city into cells based on points of interest

This discovery was a major blow to the miasma theory of disease transmission, but more important was Snow’s method for uncovering the truth about the cholera outbreak. He took an unbiased approach to collecting and interpreting data and tried to understand the geographic and cultural mechanisms at play without resorting to preconceived beliefs. His success opened the floodgates for modern day epidemiology.

John Snow’s story is just one of many in a cultural shift towards the empirical method and modern statistics.  It proved that gaining quantitative knowledge of the world can often unveil secrets that would have otherwise gone undetected.  Perhaps most importantly, it was a lesson in letting the data speak for itself.

Leave a Reply


  1. Pingback: Our Favorite Analytics Stories | Aunalytics

  2. Anonymous

    great piece of article,very siplified