Many years ago, scientists and doctors had no idea what caused diseases in people. There were theories that people who succumbed to disease had breathed bad air, had committed a bad deed, or even that they had bad blood, prompting doctors to let out some of their blood. During a cholera outbreak in London in 1854, John Snow, a London physician and scientist, mapped the location of the cholera victims. He was a skeptic of the "bad air" theory and proposed that a contaminated water source was the cause of the cholera outbreak. His map showed a cluster of early deaths around a particular water pump. The disease outbreak declined because the water pump was closed (by removing its handle) and because people had fled the area. His idea that the disease was caused by contaminated water was confirmed by an inspection of the well that discovered raw sewage had seeped into it. Which of the following is this the best example of?
