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“Oxygen! nobody knows what that may be—is it any wonder the cholera has got to Dantzic? And yet there are people who say quarantine is no good!”

This 19th-Century speculation about cholera from George Eliot’s Middlemarch shows how little anyone knew, not just about oxygen, but more significantly about the terrible cholera epidemic that first hit Europe in the 1830s. Doctors and scientists struggled to find its cause, and most believed that it was carried by “miasma,” or foul air. But in the 1850s, the English physician John Snow was the first to use disease mapping to prove that cholera is transmitted not through the air, but through water. His findings, published in Cholera and the Water Supply (1854) and On the Mode of Communication of Cholera (2nd ed. 1855), revolutionized the water and sanitation systems of London and many large cities worldwide. These important works were turned into e-books by Distributed Proofreaders and are available for free download at Project Gutenberg.

No one yet knew that cholera is caused by a bacterial infection of the small intestine; the Vibrio cholerae bacterium was not identified as the cause until 1884. But Snow was all too familiar with its symptoms: profuse diarrhea and vomiting that caused severe dehydration, which in turn often led to death by organ failure. The first known cholera pandemic began in India in 1817. The rise of international trade and travel soon brought cholera to North America and Europe in waves of outbreaks.

A two-year outbreak in England beginning in 1848 prompted Snow to publish the first edition of On the Mode of Communication of Cholera in 1849. He rejected the miasma theory. His research showed that men who worked in London’s sewers, which were full of foul air, were no more prone to contracting cholera than the average citizen. Moreover, Snow himself was frequently exposed to patients with cholera, but he never caught the disease. He concluded that there must be some other medium – most likely water contaminated by feces.

Snow was able to put his theory to the test in 1854, when an outbreak in the Soho district of London killed 600 people in one week alone. Research by another scientist had already shown that cholera cases in another district plunged after the local water company changed its source to a cleaner part of the Thames River. Encouraged by this, Snow set out to test the waterborne theory. He used government death records to identify the cholera victims’ residences in Soho. He conducted house-to-house interviews of their families and learned that they drew their water from a particular well on Broad (now Broadwick) Street, as did a number of local businesses, including pubs and restaurants that some of the victims frequented. Rich and poor alike were affected; poverty and overcrowding were clearly not the cause. His map of the outbreak (above), with black bars indicating cholera deaths, vividly proves his point.

So what was the deadly problem with the Broad Street pump? Snow learned that the well was within just a few yards of a sewer. He took a sample of the water and saw “impurities of an organic nature.” And the water developed a foul odor when stored – a hallmark of water contaminated with sewage. He concluded that cholera was spread by evacuations from cholera patients entering the sewer and permeating into the nearby well. On that basis, he convinced the local authorities to disable the pump.

Snow’s cholera study went beyond Broad Street. He also mapped the different water supplies throughout London, which showed far fewer cholera deaths in areas where the water company sourced its water from a clean part of the Thames. And he provided detailed statistics as to where cholera deaths occurred.

Cholera killed millions all over the world in the 19th Century. The mortality rate was drastically reduced in developed countries once Snow’s and other scientists’ discoveries led to changes in sanitation systems. Unfortunately, cholera remains a problem today in developing countries that still have poor sanitation, as the European Centre for Disease Prevention and Control reports. But likelihood of death is much lower than it once was, thanks to modern hydrating treatments.

Snow’s diligent research and the unique and compelling way he presented it are key events in the early development of modern epidemiology. Thanks to the volunteers at Distributed Proofreaders and Project Gutenberg, you can be present at the birth of a vital, life-saving science.

This post was contributed by Linda Cantoni, a Distributed Proofreaders volunteer.