In 1854 there was a cholera epidemic in London.
The accepted theory at the time was that illness and epidemics spread through the “miasma”, a form of “bad air”, pollution and smell emanating from decomposing organic matter. The mechanics of germs was not understood.
Dr. John Snow was sceptical to the miasma theory, but not entirely grasping germ mechanics, he did this admirable thing: scientific observation, quantitative and qualitative data collection and using the best medium for studying the results.
He talked to the residents in the area of outbreak. He then hypothesised that there was a water pump that was the source. He took water samples, but these were inconclusive. He gathered data on how many people had died and where they lived. Plotting this on a map, it seems clear that infections are concentrated around the Broad Street water pump:
However; there are inconsistencies that are a little problematic to explain, looking at the map. Armed with the quantitative data, our Dr. Snow did some more research by talking to people living at points where these oddities occur.
Quite a long way away from the Broad Street pump, closer to other pumps, some people got infected. The reasons, it turned out, being that children living there went to school closer to Broad Street, or that the people living there preferred the water from the Broad Street pump, considering it better.
Another anomaly was a few buildings close to the pump that had no infections, for example a a work house and a brewery. Some further researched showed that the work house had their own water source, and that brewery employees did not really drink water, they drank beer… So, if there was ever uncertainty; graphics are beneficial for public health.