Johnson, Steven. Ghost Map: The Story of London's Most Terrifying Epidemic -- and How It Changed Science, Cities and the Modern World, Riverhead Books, 2006.
The map in the title of this
book shows houses and wells in mid-19th-century London. In 1854 Soho experienced the most violent outbreak of cholera in the city's history. The map, compiled by the physician John Snow, shows the number of dead for each house and which well is closest in terms of walking distance. Its story is usually told as one of cartographic innovation - the map eventually helped to convince the political scientific establishment of the day that keeping drinking water clean (ie. improving and rerouting sewage systems) could prevent cholera epidemics.
Steven Johnson traces Snow's efforts and finds that the map was merely the final product of a much more fascinating story.
The accepted theories of cholera transmission at the time were a) a miasma theory that related disease to "bad air" and stench produced by unsanitary living conditions and b) that the lower classes were more susceptible to disease through unspecified "moral failings."
Snow, a prolific scientist and distinguished physician - and more importantly a rigorous empiricist, could find no support for these theories. His observations suggested that patients contracted cholera by ingesting something harmful, not by inhaling it or through a weak moral constitution. He had a hunch that cholera was spread through water contaminated by sewage and set out to prove it by linking cholera cases and different water sources.
Snow's initial focus of inquiry was an area of London that had a mixed water supply, ie. different houses received their water from different companies. At one point, he spent days going from house to interviewing residents about their water consumption. When the 1854 epidemic broke out in his own neighborhood, he not only tended to the ill, but also kept records of cholera deaths by house and linked them to a specific pump on nearby Broad Street. Again, he spent hours interviewing people about their drinking habits and also managed to link several deaths further afield to the same pump.
Snow presented his data to the Board of Governor's of St. James Parish as they tried to deal with epidemic. The board members were skeptical - after all, the Broad Street pump was known for its particularly pure water. But they had few other options, and the risks of shutting down the pump were low compared to the potential of saving tens or hundreds of lives. So, a week after the first outbreak of cholera, the handle on the Broad Street pump was removed. While this was probably the first scientifically sound reaction in the battle against cholera, the neighborhood public and the national Board of Health were not convinced.
Henry Whitehead, a clergyman, spent most of his days walking around the Broad Street neighborhood, talking with his parishioners. When the cholera epidemic hit, he saw the consequences first-hand. He soon discredited both the "miasma" and the "moral weakness" arguments. He realized that there were fewer higher-class deaths than lower-class deaths per house, simply because the lower-class apartments were more densely populated. In fact, per capita death rates were unrelated to class. One of the most severely affected houses was locally known to be one of the cleanest, unlikely to suffer more from miasma than many filthier houses that had fewer cholera cases. The local work house which should have been hit worst, reported remarkably few deaths.
When Whitehead heard of Snow's water-borne explanation, he set out to debunk it like the others. He had seen several patients recover after drinking large quantities of water from the Broad Street pump (thereby almost stumbling on the cure for cholera), so he thought that discrediting Snow's theory would be easy. However, the more he spoke to survivors and former residents who had fled the epidemic, the more supporting evidence he found. Where Snow had tallied deaths and linked them to the pump, Whitehead added survivors and linked them to alternate water sources (or beer consumption instead of water).
Eventually, it was Whitehead who discovered the source of the epidemic: by chance he stumbled on the record of the death of a baby girl, reportedly from diarrhea, who fell sick a few days before the outbreak. The girl's address was immediately next to the Broad Street pump, and the family used a (officially nonexistent) cesspool located just a few feet from the pump for their household waste. By now, Snow's and Whitehead's case was convincing enough to warrant an inspection of the well. The final confirmation came when the cess pool was found to be leaking into the well.
These results were reported by the vestry of St. James parish, and both Snow and Whitehead wrote extensively about the cholera outbreak in the following years. Still, the scientific community clung to miasma and class/morals-based theories. The official report of the national Board of Health blamed miasma for the 1854 epidemic and all but ridiculed Snow's water-borne theory.
And finally, we come to the map. Snow started working on it several months after the outbreak. Several maps had been created to analyze the cholera epidemic, but these weren't linked to rigorous scientific theorizing and data. By reducing the map to streets, houses, pumps and cholera deaths, Snow made a striking visual case for his theory. A revised version was included in the St. James vestry's report, and this contains the most innovative technique used on the map: a dividing line shows groups houses together in terms of which well is closest by walking distance (a so-called Voronoi diagram). The geography of Soho streets and alleys meant that walking time and distance as the crow flies were not always correlated. This version posed an even more striking argument linking the Broad Street pump to the 1854 cholera epidemic.
While the map was important, Johnson shows that it - and the theories it supported - would never have seen the light of day without the incredible depth of local, even amateur, knowledge that Snow and Whitehead brought to it. And while the map legitimized the first correct science-based response to a cholera epidemic, its authority was based in the scientific inquiry behind it.
Johnson also argues that the epidemic was as much a product of the city as the ultimate measures to prevent it. Cholera could never have spread as virulently without the incredible population density found in London in the 1850s; it would never have found a way to spread if London hadn't been overwhelmed by the sheer quantity of human waste produced by this population. On the other hand, two working class men (John Snow and Henry Whitehead) would hardly have gained the education and reputation to influence public health policy anywhere outside the socially mobile city; high population density also meant that Snow and Whitehead were able to gather enough data to convincingly prove their theory.
The massive reconstruction of the sewage system that followed many years later (and only after another decimating epidemic) essentially rid London of cholera. This example, not only allowed other cities to improve their sanitation cities, it also proved the enduring viability of the city. Widespread ideas that cities of a million of more people were destined to self-destruct and drown in their own waste were refuted once and for all.
Links:
Online resources for the book (incl. links to the map and the UCLA department of epidemiology's John Snow site).
Steven Johnson's
TED talk on the cholera epidemic.
The
map (without the Voronoi diagram showing walking time).
Review of Johnson's first book, "Interface Culture."
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The Ghost Map: The Story of London's Most Terrifying Epidemic--and How It Changed Science, Cities, and the Modern World
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