John Snow was a medical doctor who mapped out an outbreak of cholera in 1854 London. His map helped him figure out the cause of cholera was water based (the point of origin of the outbreak was a water pump's well) rather than the "bad air" of urban living which was the accepted scientific cause at the time.
Snow's use of geographical thinking started the field of medical geography. Countless lives have been saved by his research and spin-offs from it.
John Snow and Geography: Knew It, Mapped It, and Still Stops Epidemics Around the World.