In mid-1800s London, an anesthesiologist named John Snow conducted a series of field investigations that have earned him the title the “father of field epidemiology.” Snow conducted studies of cholera outbreaks and discovered the cause of disease and prevented its recurrence. In 1854, there was a cholera epidemic in the Golden Square neighborhood of London. He began his investigation by determining where in the area people with cholera lived and worked. He marked each residence on a map. Because Snow believed that water was a source of infection for cholera, he marked the location of water pumps on his map, then looked for a relationship between the distribution of households with cases of cholera and the location of pumps. Even with no knowledge of the existence of microorganisms, Snow demonstrated through epidemiologic studies that water could serve as a vehicle for transmitting cholera and that epidemiologic information could be used to direct prompt and appropriate public health action as recourse and prevention.