Snow provided evidence linking a specific water source to cholera by showing an association between drinking from which source and getting cholera?

• A. The Broad Street well water and cholera
• B. The River Thames water and cholera
• C. City reservoir water and cholera
• D. Bottled water and cholera

Answer: (A)

The main idea being tested is how epidemiologists link a disease to a specific exposure by comparing who becomes ill with who is exposed, then using an intervention to see if disease transmission changes. John Snow showed that cholera was connected to a particular water source by mapping cases and asking people where they obtained their drinking water. He found that those who drank from the Broad Street pump had many more cholera cases, while neighbors who used other water sources were less affected. When the pump handle was removed, the outbreak declined, which strongly supported that the water from that Broad Street source was the vehicle for transmission. This concentrated, source-specific association and the subsequent drop in cases after removing the exposure provide the strongest evidence that the Broad Street well water was the source of the outbreak. The River Thames, city reservoir, or bottled water did not show the same consistent link in his data, so they aren’t the sources responsible for this outbreak.