Shanghai. New York. Tehran. Tokyo. Today, dozens of cities worldwide are each home to many millions of people. But those masses of humanity might not exist in such tight quarters if not for John Snow. (No, not that Jon Snow. This John Snow.)
Snow was a 19th-century English doctor who's credited with proving that cholera, a sometimes deadly infection that attacks the small intestine, spreads through contaminated water -- and not by “bad air” as was generally believed at the time.