A little of history:

The Broad Street cholera outbreak (or Golden Square outbreak) was a severe outbreak of cholera that occurred in 1854 near Broad Street in London, England. This outbreak, which killed 616 people, is best known for the physician John Snow's study of its causes and his hypothesis that germ-contaminated water was the source of cholera, rather than particles in the air (referred to as "miasmata"). *

Snow relied on visualizing the streets along with the water pumps of where most deaths occurred, which helped him reach this resolution.