Death on Broad Street
In late August 1854, a swift illness descended upon the Soho district of London. Within three days, 127 people living around Broad Street were dead. Over the course of just 10 days, the death toll from this cholera outbreak would climb to over 500, eventually claiming 616 lives. The prevailing scientific theory of the day held that diseases like cholera were spread by "miasma," a form of bad air rising from filth and decay. Most physicians believed this foul vapor was the killer in Soho's residents.
A local physician and anaesthetist named John Snow was skeptical of the miasma theory. Having witnessed earlier cholera outbreaks, he had published a paper in 1849 arguing that the disease was not airborne but was ingested orally, likely through contaminated water. The Soho outbreak presented an opportunity for Snow to test his hypothesis. He began a methodical investigation, going from house to house to interview residents and document the deaths.
The Ghost Map
Snow plotted each death on a map of the neighborhood, with a small black bar representing each fatality at its address. The map, which became known as the "Ghost Map," showed a pattern: the deaths were clustered most densely around the public water pump on Broad Street (now Broadwick Street). His investigation uncovered anomalies that supported his waterborne theory. A nearby workhouse had its own private well and suffered very few cholera deaths. Workers at the local brewery also remained largely unscathed; they were given a daily allowance of beer and drank little water from the pump.
A piece of evidence came from outside Soho. A widow who had moved to Hampstead, several miles away, had died of cholera. Snow learned that she had a particular fondness for the taste of the water from the Broad Street pump and had it delivered to her home daily. Her niece, who visited and also drank the water, fell ill as well. With this evidence, Snow presented his findings to the local parish Board of Guardians on September 7, 1854. Although skeptical, they agreed to remove the pump handle the next day, September 8. Following this action, the number of new cases in the area dropped dramatically.
A later investigation, assisted by a local curate named Henry Whitehead, discovered the source of the contamination. The well for the Broad Street pump had been dug only a few feet from a leaking cesspool. Into this cesspool, the mother of a baby who had died from cholera at the very start of the outbreak had been emptying soiled nappies. This confirmed the well was contaminated with the bacterium now known as Vibrio cholerae, a microbe that would not be formally identified until 1883 by Robert Koch.