He’s a guy that’s often thought of as the founder of modern day epidemiology because of his work in tracing the source of a cholera outbreak in England in the mid 1800s. At the time, scientists and the public believed that diseases like cholera were spread by “miasma”- basically that diseases like cholera were spread by pollution or “bad air”. The “germ theory” of disease was in its infancy, so Snow didn’t have the advantage of knowing that microorganisms spread disease. But he was skeptical that bad air was the cause.
During an outbreak of cholera in the mid 1860s he started talking to local residents and concluded that the source of the outbreak might actually be from the water in a public water pump on Broad Street. He didn’t have the instruments to observe a problem with the water but the pattern of disease he documented was convincing enough to persuade the local council to disable the well pump by removing its handle. He later developed a spot map to illustrate how cases of cholera clustered around the pump and statistically showed the connection between the quality of the source of water and cholera cases.
Looking behind the scenes, he found that homes that had an increased incidence of cholera were more likely to be delivered water from the parts of the Thames River that were polluted with sewage. At the time, it was a really hard sell to convince the public that many diseases were spread through the fecal-oral route partly because it grossed people out. However, the next decade (1860s-1870s) was a real growth period for public health as the germ theory developed, opening the possibilities for hundreds of public health interventions.