Read the passage about Dr. John Snow. In 1849, doctor John Snow began to question the long-held beliefs that disease was caused by “bad air.” When a cholera epidemic broke out in London, he interviewed residents of the hardest-hit areas. He mapped cases of the disease and determined that they were centered around a particular water pump. To test his theory that contaminated water was to blame, he convinced local leaders to disable the pump by removing the pump handle. The outbreak came to an end. What was Dr. Snow’s hypothesis? Cholera was caused by “bad air.” Cases were centered around a water pump. Contaminated water caused cholera. Leaders should disable the water pump.