Today I learned that a map by John Snow defeated a cholera outbreak in the middle of London by tracking down cases and carefully mapping them out to proof his theory that cholera was transmitted by water. In the 19th century there was a world-wide spreading cholera pandemic. The around that time common “Miasma theory” was considered as explanation for the spread of cholera, even after John Snow was able to proof that the main transmission took place through water by locating a polluted water source in the street, getting it taken out of service by the city, which drastically dropped case numbers in the area, and additionally explain why anomalies in his map data even appeared in first place.
I came across this by watching a “Map men” video, which I consider a quite entertaining and informative format about maps and their influence on (mainly British) societies. And of course by reading all the Wikipedia article linked above, which provided me with a bit more insight about that pandemic of the 19th century and the Miasma theory.