To prove his case, Snow visited every house in the area to ask about its water supply. It was a frustrating business. "There were very few instances [where] I could at once get the information I required," he noted.
Eventually, Snow uncovered a pattern. While 18 workers at one factory had died of cholera, none in the brewery next door was affected. Crucially, the latter did not drink local water. They had free beer.
Snow presented his data on a map. The result was striking. The closer a residence was to the water pump in Broad Street (now called Broadwick Street), the greater the number of deaths – with a few telling exceptions coming from properties that had their own water sources.
At Snow's insistence, the handle of the Broad Street pump was removed, preventing locals from drinking its contaminated water. Legend has it that the outbreak then died out. In fact, it had already peaked and it took several more years before authorities fully accepted the case that cholera was waterborne.
A nearby cesspit was leaking faeces into the pipes that supplied the Broad Street pump, we now know. Crucially, Snow examined that pump water expecting to see organic contamination.
The water looked pure and clear, however. The idea that tiny, invisible bacteria were the causes of diseases such as cholera had yet to be demonstrated by Pasteur and Koch. Nevertheless, Snow remained sure of his case, largely because his collection of data had been so sound.
Equally, his presentation of his findings in map form made his conclusions all the more convincing. There is nothing to beat good data that is well presented, we now know.
We therefore have much to thank Snow for. You can even visit a recreation of the Broad Street pump and then celebrate his life with a pint in the nearby John Snow pub – though what the great man would have thought is unclear. He was, it transpires, a teetotaller.