In the nineteenth century, London was a powerful city, in full industrial revolution, but overcrowded, with serious problems of dirt and equipped with primitive sewage networks, which threw the drains into the Thames and took drinking water from the same river. It was this action that contaminated the city's water supply, leading to cholera epidemics.
At the time of the outbreak of the cholera epidemic of 1854, studies conducted by the English doctor John Snow with an epidemiological method for the innovative era, were able to show that cholera was transmitted by water and not by air, as until then he supported the theory of miasmas . To demonstrate his thesis, Snow decided to compile " ghost maps ": he took some maps showing the public wells and all the known deaths from cholera, in correspondence of which he drew black bars stacked perpendicular to the roads. The more signs were marked on the map, the greater the number of deaths at that particular point in the city. Monitoring the Soho district in particular, the doctor found that a large number of deaths occurred near the water pump located on Broad Street. Snow removed the handle of the fountain and the cases of cholera in the neighborhood began to decline, only to run out in a few days. In practice, John Snow with his method succeeded in linking the incidence of cholera and its diffusion in a precise geographical area, therefore to the most probable vehicle of diffusion.
