John Snow was a British physician who helped change how people investigate disease outbreaks. In 1854, a deadly cholera outbreak struck the Broad Street area of London, where many people still believed disease spread through bad air. Snow studied where people lived, where they got water, and where deaths occurred.
His work matters because it showed that careful evidence, maps, and data can reveal the source of a public health crisis.
Snow noticed that cholera deaths clustered around a public water pump on Broad Street. By comparing cases, water sources, and neighborhood patterns, he argued that contaminated water was spreading the disease. The pump handle was removed, and the outbreak soon declined, although the epidemic may already have been slowing.
Snow's investigation became a foundation of modern epidemiology, disease mapping, and environmental health science.
Key Facts
- John Snow lived from 1813 to 1858 and worked as a physician in London.
- The 1854 Broad Street cholera outbreak was linked to contaminated water from a public pump.
- Epidemiology studies patterns, causes, and control of disease in populations.
- Attack rate = number of new cases / number of people at risk.
- Incidence rate = new cases during a time period / population at risk during that time period.
- Snow's map connected disease cases to location, making spatial data a tool for public health decisions.
Vocabulary
- Epidemiology
- Epidemiology is the study of how diseases spread through populations and how they can be controlled.
- Cholera
- Cholera is a bacterial disease that can cause severe diarrhea and dehydration, often spread through contaminated water.
- Waterborne disease
- A waterborne disease is an illness transmitted when people drink or use water contaminated with harmful organisms or substances.
- Disease mapping
- Disease mapping is the use of maps to show where cases occur and to identify patterns in an outbreak.
- Outbreak
- An outbreak is a sudden increase in disease cases above what is normally expected in a place or group.
Common Mistakes to Avoid
- Assuming Snow proved cholera bacteria existed, which is wrong because he identified the transmission pattern before the bacterium was widely accepted as the cause.
- Thinking the pump handle removal alone immediately ended the outbreak, which is too simple because cases were already declining and many factors affected transmission.
- Confusing correlation with causation, which is wrong because Snow used multiple lines of evidence, including location, water source, and comparison groups, to support his conclusion.
- Ignoring population size when comparing neighborhoods, which is wrong because more cases in a larger population may not mean a higher risk.
Practice Questions
- 1 In one block near the Broad Street pump, 48 cholera cases occurred among 600 residents. Calculate the attack rate as a decimal and as a percent.
- 2 A nearby area had 18 cholera cases among 900 residents. Compare its attack rate with the block that had 48 cases among 600 residents. How many times higher was the first attack rate?
- 3 Explain why mapping cholera deaths around water pumps was stronger evidence than simply counting the total number of deaths in London.