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Tuesday, December 4, 2007

Reading List: The Ghost Map

Johnson, Steven. The Ghost Map. New York: Riverhead Books, 2006. ISBN 1-59448-925-4.
From the dawn of human civilisation until sometime in the nineteenth century, cities were net population sinks—the increased mortality from infectious diseases, compounded by the unsanitary conditions, impure water, and food transported from the hinterland and stored without refrigeration so shortened the lives of city-dwellers (except for the ruling class and the wealthy, a small fraction of the population) that a city's population was maintained only by a constant net migration to it from the countryside. In densely-packed cities, not only does an infected individual come into contact with many more potential victims than in a rural environment, highly virulent strains of infectious agents which would “burn out” due to rapidly killing their hosts in farm country or a small village can prosper in a city, since each infected host still has the opportunity to infect many others before succumbing. Cities can be thought of as Petri dishes for evolving killer microbes.

No civic culture medium was as hospitable to pathogens as London in the middle of the 19th century. Its population, 2.4 million in 1851, had exploded from just one million at the start of the century, and all of these people had been accommodated in a sprawling metropolis almost devoid of what we would consider a public health infrastructure. Sewers, where they existed, were often open and simply dumped into the Thames, whence other Londoners drew their drinking water, downstream. Other residences dumped human waste in cesspools, emptied occasionally (or maybe not) by “night-soil men”. Imperial London was a smelly, and a deadly place. Observing it first-hand is what motivated Friedrich Engels to document and deplore The Condition of the Working Class in England.

Among the diseases which cut down inhabitants of cities, one of the most feared was cholera. In 1849, an outbreak killed 14,137 in London, and nobody knew when or where it might strike next. The prevailing theory of disease at this epoch was that infection was caused by and spread through “miasma”: contaminated air. Given how London stank and how deadly it was to its inhabitants, this would have seemed perfectly plausible to people living before the germ theory of disease was propounded. Edwin Chadwick, head of the General Board of Health in London at the epoch, went so far as to assert (p. 114) “all smell is disease”. Chadwick was, in many ways, one of the first advocates and implementers of what we have come to call “big government”—that the state should take an active role in addressing social problems and providing infrastructure for public health. Relying upon the accepted “miasma” theory and empowered by an act of Parliament, he spent the 1840s trying to eliminate the stink of the cesspools by connecting them to sewers which drained their offal into the Thames. Chadwick was, by doing so, to provide one of the first demonstrations of that universal concomitant of big government, unintended consequences: “The first defining act of a modern, centralized public-health authority was to poison an entire urban population.” (p. 120).

When, in 1854, a singularly virulent outbreak of cholera struck the Soho district of London, physician and pioneer in anæsthesia John Snow found himself at the fulcrum of a revolution in science and public health toward which he had been working for years. Based upon his studies of the 1849 cholera outbreak, Snow had become convinced that the pathogen spread through contamination of water supplies by the excrement of infected individuals. He had published a monograph laying out this theory in 1849, but it swayed few readers from the prevailing miasma theory. He was continuing to document the case when cholera exploded in his own neighbourhood. Snow's mind was not only prepared to consider a waterborne infection vector, he was also one of the pioneers of the emerging science of epidemiology: he was a founding member of the London Epidemiological Society in 1850. Snow's real-time analysis of the epidemic caused him to believe that the vector of infection was contaminated water from the Broad Street pump, and his persuasive presentation of the evidence to the Board of Governors of St. James Parish caused them to remove the handle from that pump, after which the contagion abated. (As the author explains, the outbreak was already declining at the time, and in all probability the water from the Broad Street pump was no longer contaminated then. However, due to subsequent events and discoveries made later, had the handle not been removed there would have likely been a second wave of the epidemic, with casualties comparable to the first.)

Afterward, Snow, with the assistance of initially-sceptical clergyman Henry Whitehead, whose intimate knowledge of the neighbourhood and its residents allowed compiling the data which not only confirmed Snow's hypothesis but identified what modern epidemiologists would call the “index case” and “vector of contagion”, revised his monograph to cover the 1854 outbreak, illustrated by a map which illustrated its casualties that has become a classic of on-the-ground epidemiology and the graphical presentation of data. Most brilliant was Snow's use (and apparent independent invention) of a Voronoi diagram to show the boundary, by streets, of the distance, not in Euclidean space, but by walking time, of the area closer to the Broad Street pump than to others in the neighbourhood. (Oddly, the complete map with this crucial detail does not appear in the book: only a blow-up of the central section without the boundary. The full map is here; depending on your browser, you may have to click on the map image to display it at full resolution. The dotted and dashed line is the Voronoi cell enclosing the Broad Street pump.)

In the following years, London embarked upon a massive program to build underground sewers to transport the waste of its millions of residents downstream to the tidal zone of the Thames and later, directly to the sea. There would be one more cholera outbreak in London in 1866—in an area not yet connected to the new sewers and water treatment systems. Afterward, there has not been a single epidemic of cholera in London. Other cities in the developed world learned this lesson and built the infrastructure to provide their residents clean water. In the developing world, cholera continues to take its toll: in the 1990s an outbreak in South America infected more than a million people and killed almost 10,000. Fortunately, administration of rehydration therapy (with electrolytes) has drastically reduced the likelihood of death from a cholera infection. Still, you have to wonder why, in a world where billions of people lack access to clean water and third world mega-cities are drawing millions to live in conditions not unlike London in the 1850s, that some believe that laptop computers are the top priority for children growing up there.

A paperback edition is now available.

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