One of the most famous stories in all of epidemiology revolves around the very birth of the science, in the midst of a London cholera outbreak in 1854. At the time, the scientific community was divided over the cause of cholera and other diseases. The majority of them accepted the miasma theory, the idea that disease was due to corrupted air (“all smell is disease,” noted sanitation commissioner Edwin Chadwick). This idea dates back to antiquity, and increased in popularity in the Victorian era. It’s a great example of something that logically made sense, even though it was wrong. 19th century sanitation reformers pointed out to disease outbreaks that occurred in areas that were filthy, and along with that filth came a terrible smell. It was thought that the scent was due to the putrefaction of the air, and that when this putrid air was inhaled, it resulted in the development of disease. Again, it made sense–when areas were cleaned up, disease frequently decreased–it seemed like a no-brainer. Though this was prior to the formulation of the germ theory of disease, some scientists (dubbed “contagionists”) believed disease was not acquired via miasma, but instead passed from person to person via some sort of unidentified particle.

Physician John Snow was one of the early supporters of the latter theory. His careful investigation of the 1854 London cholera outbreak was the beginning of the end of the miasma theory–and the beginning of modern epidemiology as well. However, the impact of the cholera epidemic extends much farther than just the eventual formulation of the germ theory of disease and the downfall of the miasma theory. In The Ghost Map, Steven Johnson tells the fascinating history of John Snow’s groundbreaking investigation, and how it still reverberates in the world today.

The Ghost Map is mostly a story of the cholera investigation, and especially John Snow’s role in it. At the time, Snow was already a noted physician, famous for his studies on ether and chloroform anaesthesia. He carefully determined dosages of these chemicals that could be used in surgery and during childbirth, and was so successful that Queen Victoria even had him attend to the birth of her son. However, Snow wasn’t content to limit himself to one area of study. Instead, he published research in a wide variety of areas–writing about lead poisoning, smallpox, scarlet fever, and the circulatory system, among others. He also long held a fascination with cholera, first discussing that topic in a pair of manuscripts in 1849. These documented a cholera outbreak that began the previous year, and left 50,000 dead by the time it ended. In these papers, Snow advanced his theory about the cholera’s cause and transmission, suggesting 1) that the disease was transmitted by water, not air; and 2) that it was due to some as-yet-unidentified agent present in the water. So yes, cholera was a disease of filth, but it was due to dirty water, not fetid air.

Not surprisingly, Snow encountered much disagreement with his ideas. He knew that, while his work was solid and it showed a convincing correlation between water and cholera, he’d not proved causation. What he needed was, as a newspaper suggested, an experimentum crucis:

The experimentum crucis would be, that the water conveyed to a distant locality, where cholera had been hitherto unknown, produced the disease in all who used it, while those who did not use it, escaped.

The 1854 outbreak, then, allowed him to further gather data and test his hypothesis–and perhaps obtain his experimentum crucis. Snow, in fact, was ideally suited for this, for a number of reasons. First, his investigations into previous cholera outbreaks taught him how to use demographic data and statistics and relate those to an epidemic. Two, the outbreak just happened to take place in his own neighborhood. This allowed him to do a great deal of first-hand detective work, interviewing cases (or relatives, friends, and neighbors of victims who had died), finding common threads that ran through the stories not only of those who became ill, but also of those that remained well. Though the outbreak was concentrated in his neighborhood, residents in some buildings remained free of disease, while those who lived next door were highly affected. Why? One example was a brewery, where scarcely any cases of cholera had been reported. When Snow asked about the water the employees drank, he found they rarely drank water–they drank malt liquor instead. In another nearby factory, however, a high proportion of workers had become ill or died–and Snow found they gave their employees water from a pump he’d already suspected was at the root of the outbreak. Armed with data about cases of disease and supply of drinking water in the neighborhood, Snow finally felt he had proved his case. Slowly, others would eventually be won over.

While John Snow’s story is the central thrust of The Ghost Map, Johnson does just what Christine Gorman suggested journalists should do–he looks at it from all levels, from the microscopic (the causative organism, Vibrio cholerae), to the macroscopic (sanitation, sewage, the very infrastructure of cities). All played a role–and continue to play a role–in cholera outbreaks. Though many in public health will already be familiar with the general story of John Snow (and others who played a role, including Reverend Henry Whitehead, who pinned down the index case that began the outbreak), Johnson does much more than just re-tell Snow’s story. He puts it in the much larger context of how we deal with sanitation in general, and how that continues to affect urban planning 150 years after Snow’s research. Sadly, he also demonstrates that in parts of the world today, little has changed since John Snow’s London.

Further reading

Steven Johnson, The Ghost Map. Riverhead Books: 2006.

All about John Snow, UCLA Department of Epidemiology.

Image from http://www.york.ac.uk/depts/maths/histstat/images/snow_map.gif