What did Snow's Broad Street cholera investigation conclude about the source of the outbreak?

• A. Air pollution
• B. Immune deficiency
• C. Starvation
• D. Contaminated water

Answer: (D)

Cholera spreads through water contaminated with sewage, so you test whether cases cluster around a water source rather than scattered randomly through the air. In the Broad Street study, Snow mapped who got sick and where they got their water. He found a strong link between illness and using the pump on Broad Street, and he traced the supply to a water source that could be tainted by nearby sewer leakage. When the pump handle was removed, the outbreak waned, which created a clear cause-and-effect signal for waterborne transmission. This contrasted with the idea that the disease came from bad air, or air pollution. It also makes less sense to attribute the outbreak to immune deficiency or starvation, which wouldn’t produce such a sharp, localized pattern or respond to removing a single water source. So the outbreak source was contaminated water.