Around 400 B.C.E., the Athenian physician Hippocrates catalogued the diseases of his world. Cholera was on the list. But while Hippocrates provides the first proof of cholera beyond a reasonable doubt, the disease likely originated along the Ganges River while Athens was still a very young place.
Cholera lives in many of the world's water sources, but it's most dangerous when it has an environment in which there are many people among whom it can spread. The Ganges River happens to be one of the most ancient locations of human population density, and so it was long, long ago that upstream users gathered in the numbers needed to pollute the water for those downstream. In other words, as more people become infected with cholera, they pollute the water supply with more bacteria, which in turn infects more people.
Interestingly, the same problem might have been a major factor in the loss of troops in Hannibal's march across the Alps. With a 50,000-soldier train, the troops and animals in front would have encountered pristine mountain streams, but those in back would have been forced to deal with putrid and potentially cholera-rich water [source: Hunt].