Which of the following was a contribution of John Snow that helped develop epidemiologic thinking?

• A. He proposed a new hypothesis for how cholera was transmitted.
• B. He developed a vaccine against cholera.
• C. He discovered the bacterium causing cholera.
• D. He implemented mass quarantine measures.

Answer: (A)

Forming a testable hypothesis about how a disease spreads from population observations is a foundational habit in epidemiology. John Snow contributed by proposing that cholera is transmitted through contaminated water, not simply by foul air or unknown causes. He gathered and mapped data on cholera cases around London and noticed clusters linked to the public water source used by those households. This pattern suggested a specific exposure route rather than a vague environmental factor. He then tested the idea in a practical way by removing the Broad Street pump handle, which coincided with a drop in new cases. That intervention-level observation provided strong support for the waterborne transmission hypothesis and demonstrated how identifying a transmission mechanism could guide effective public health actions. This way of linking disease patterns to a plausible transmission route and testing it with an intervention helped establish how epidemiologists think about and study infectious diseases. He did not develop a cholera vaccine, discover the bacterium, or implement mass quarantines, so those options don’t reflect his actual contribution.