Skip to main content
A colourised image of John Snow

The 1854 Broad Street cholera outbreak

Explore how John Snow's investigation of the 1854 cholera outbreak in Soho revolutionised the science of disease control.

Image: Physician John Snow findings laid the groundwork for modern epidemiology | Public Domain

Also known as the ‘Golden Square outbreak’, the 1854 Broad Street cholera outbreak remains one of the most important events in the history of public health, health geography and epidemiology.

In 1854, a serious outbreak of cholera near Broad Street (now Broadwick Street) in Soho, London, killed 616 people. Physician John Snow’s investigation of the outbreak critically changed our understanding of how infectious diseases spread, ultimately leading to the development of modern epidemiology.

Background to the 1854 Broad Street cholera outbreak

In the 19th century, cholera was a major health threat causing severe diarrhoea and dehydration that could lead to death if untreated.

London was a rapidly growing city throughout the 1800s and was no stranger to plagues and epidemics. However, the city lacked the adequate sanitation and sewage systems needed to accommodate this population growth. The area of Soho was particularly prone to contaminated water as the city’s sewer system had not been extended to the borough.

Excrement from cowsheds and slaughterhouses, combined with street pollution and overflow from cesspools, polluted the water supply. This led to a series of serious cholera outbreaks between the 1830s and 1850s.

The Golden Square outbreak

On 31st August 1854, a serious outbreak of cholera occurred in London. By 10th September, over 500 people had died, with other residents of the area fleeing.

The severity and rapid progression of the disease caught the attention of physician John Snow who became determined to investigate the outbreak.

Medical knowledge on infectious disease was still limited, with one prevailing theory on cholera. Miasma theory hypothesised that cholera was caused by bad particulate matter (‘miasma’) in the air.

Snow’s investigation and analysis

Snow was methodical and empirical in his conduct. He collected data on the affected individuals and their water sources. This research revealed a significant cluster of cases around the Broad Street water pump, where affected individuals had either drunk water or eaten food prepared with its water.

Snow conducted this research through a two-pronged approach. He interviewed residents and found anecdotal evidence supporting his hypothesis.

Most notably, he discovered that none of the workers in the Broad Street brewery had contracted cholera. These workers were given a daily allowance of beer. Subsequently, they didn’t drink the Broad Street pump water, and any water used in the brewing process was boiled, killing any cholera bacteria in the process.

Snow also used a now-famous dot map to correspond the recorded cases of cholera with the geographic location of each affected individual. This map – which was based on what is now called a Voronoi diagram – clearly showed the highest incidences of cholera in areas of Soho that relied on the Broad Street water pump.

He subsequently proposed that affected individuals contracted cholera by ingesting contaminated water from the pump, rather than through the prevailing bad air theory.

On 7th September, Snow presented his findings to the local government, who agreed to remove the handle of the Broad Street pump. Locals could no longer use the pump as their water source, and cholera cases rapidly reduced as a result.

It was later found that the public well supplying this pump had been dug less than a metre away from an old cesspit, which was leaking into the water.

Aftermath of the outbreak and Snow’s legacy

Snow’s findings were not immediately accepted by his peers. Government officials also rejected his theory, on the basis that it would require significant changes to sanitation. This stalling no doubt contributed to the event known as the ‘Great Stink’ in the summer of 1858.

Despite immediate scepticism, however, Snow’s findings laid the groundwork for modern epidemiology. He is widely recognised as a founder of the discipline.

His investigation demonstrated the importance of rigorous, methodical statistical analysis to support evidence-based public health science. This introduction of scientific methodology for studying and controlling infectious diseases marked a departure from the speculative theories that had typically dominated the medical field.

His findings also led to eventual improvements in urban sanitation and water supply systems. A reinforced General Board of Health rolled out public health reforms that significantly reduced incidences of waterborne diseases.

These reforms and institutional changes included the construction of London’s sewer systems in the late 19th century. They contributed to the development of public health infrastructure and policy that continues to be foundational to this day.

Meanwhile, in 1992, a replica pump was installed at the site of the 1854 pump, paying tribute to Snow’s discovery and legacy.