PARIS—A hundred and fifty years ago, a maverick British physician hauled medicine into the modern era and in so doing dealt cholera one of its mightiest blows.By the mid-19th century, cholera was a byword for terror in London.Brought to Britain in 1831 from Asia, the disease erupted again and again in a city where housing was cramped and filthy, the streets awash with fetid matter and the sewers decaying where they even existed.Each outbreak would harvest thousands of lives, leaving doctors ever more befuddled about how to combat “miasma”—the foul gases that, bizarre as it may seem for us today, were blamed for the disease.The man who changed all that was a London anesthetist, John Snow.Shy, quiet but dogged, Snow had for years been ridiculed by the medical establishment for insisting that, from his own observations, cholera could only be a waterborne, not an airborne, disease.In the late summer of 1854, London was reeling in its third outbreak of cholera in 23 years.On August 31, what Snow would later describe as “the most terrible outbreak of cholera which ever occurred in the kingdom” erupted in Soho, now the heart of London’s West End.In little more than a week, 500 people died and in some streets, one in eight perished.Snow, who lived in the neighborhood, immediately began to interview families of the dead to probe the source of the problem.He drew a map of cases, which pointed overwhelmingly to a well in Broad Street (later renamed Broadwick Street) where local people drew their water.“I found that nearly all the deaths had taken place within a short distance of the pump,” he wrote.Snow then took a water sample from the pump, and using a microscope that by today’s standards would be ludicrously primitive, found “white, flocculent particles”—impurities that were clearly to blame.He did not know this, but he was seeing evidence of the Vibrio cholerae microbes that were only identified 29 years later by the German scientist Robert Koch. We now know that these germs lodge in the lower intestine, secreting toxins that cause massive fluid loss.Snow took his case to the local parish board, which on September 8 reluctantly agreed to remove the pump handle as an experiment. Within days, the outbreak was dramatically halted.Further investigations by Snow found that two women eight kilometers (five miles) away in Hampstead had been killed by the tainted water, which they had had delivered to them in bottles. And he worked with the local vicar, Reverend Henry Whitehead, to backtrack to the cause of the outbreak.The likely source was a child in Broad Street whose nappies had been washed. The washwater was then tipped into a cesspool just a yard (meter) from the well, thus seeping into the water supply.Snow wrote up his conclusions in a 139-page treatise, “On the Mode of Communication of Cholera,” and paid for its printing out of his own pocket.It took time for his arguments to change the medical establishment, but eventually they had a seismic effect that endures today.Snow rammed home the importance of hygiene in public health, prompting the London authorities to build a network of water treatment plants and sewers that became one of the wonders of the 19th century.The last outbreak of cholera in London was in 1866, although the disease remains a problem in developing countries.“Snow is an unsung hero. It’s largely thanks to him that we have managed to remove harmful bacterial from water supplies,” said Ros Stanwell-Smith of the John Snow Society in London, which has 1,100 members worldwide.The case of the Broad Street Pump also became the foundation of modern epidemiology.It is cited even today as the model for probing a disease outbreak. Snow diligently collected data, plotted cases geographically and took samples for lab analysis.He applied neutral, science-based judgment, rather than prejudice and hearsay.Snow, who died aged only 45, is virtually unrecognized by the world at large but his name enjoys great prestige amongst public health practitioners and epidemiologists.At least a dozen biographies have been devoted to him. Several countries have set up appreciation societies and their members organize lectures on drinking water safety and public health, and hand out “Pump Handle Awards” to the deserving.A 2003 poll among readers of a British magazine, Hospital Doctor, named Snow as the greatest doctor of all time. He even beat out Hippocrates, considered the founder of medicine.