John Snow and the 1854 Broad Street Cholera Outbreak: Birth of Epidemiology
In short: London physician John Snow mapped cholera deaths in Soho, traced them to a contaminated water pump on Broad Street and persuaded officials to remove its handle — a founding story of epidemiology.
The miasma belief
In the 1850s most doctors believed cholera spread through 'miasma' — bad air from rotting matter. Snow doubted this and suspected that cholera was spread by something in drinking water.
The map
When cholera killed hundreds of people around Broad Street in August–September 1854, Snow recorded where victims lived. Deaths clustered around one public water pump. Exceptions supported his theory: brewery workers who drank beer and workhouse residents with their own well were largely spared.
The natural experiment
Snow also compared households supplied by two water companies — one drawing water from a sewage-polluted part of the Thames and one from upstream. Death rates were much higher among customers of the polluted supply. This 'Grand Experiment' is still taught as a classic study design.
Lessons for today
Snow's method — count cases, map them, form a hypothesis, compare exposed and unexposed groups, and act — is exactly how outbreak investigation teams in Nepal respond to cholera and diarrhoea outbreaks in the Kathmandu Valley and Terai today.
Key takeaways
- Snow is regarded as the father of modern epidemiology.
- Spot maps and comparison groups remain core outbreak tools.
- Safe water is still the most important cholera prevention measure.
Common questions
Did removing the pump handle end the outbreak?
The outbreak was already declining, but the action symbolised evidence-based public health intervention.
What causes cholera?
The bacterium Vibrio cholerae, identified by Filippo Pacini in 1854 and later by Robert Koch in 1883.
References
Educational content by PHC-Nepal. It does not replace advice from a qualified health professional.