The Ghost Map: The Story of London’s Most Terrifying Epidemic—and How It Changed Science, Cities, and the Modern World, by Steven Johnson (Riverhead Books, 320 pp., $26.95)

However terrifying modern epidemics may appear, we have one source of comfort unavailable to our forefathers: that the scientific authorities will soon discover the cause of the malady. As many have remarked, the ability to name an enemy reduces the fear that he—or it—inspires. That is why the atmosphere that Kafka created is so disturbing: we can never identify the source of the menace.

New epidemic diseases, even if they kill fewer people overall than the old ones, are particularly frightening, making it almost impossible for people to proportion their anxiety according to the objective risks. Though cholera was not one of the greatest killers of the nineteenth century (tuberculosis easily took the palm in Western Europe), it was new and unfamiliar when it first reached Britain in 1831. The suddenness of the epidemics it caused provoked panic; but by the time Robert Koch proved the bacteriological cause in 1883, cholera had not shown up in Britain for nearly 20 years.

Not bacteriology but epidemiology and sanitary engineering defeated cholera. The 1854 cholera epidemic in London, and the subsequent removal of the handle of the Broad Street pump by John Snow, was to epidemiological history what the Declaration of Independence was to American history.

Snow was one of the most remarkable figures in medicine, though probably few know his name today, partly because almost no anecdotes survive about his character, which was retiring rather than flamboyant. The son of northern working-class parents, he managed to obtain a medical education through a combination of persistence and natural brilliance. He invented the first reliable anesthetic apparatus capable of delivering measured doses of gas to patients, thereby making the whole procedure much safer. He swiftly became the foremost authority on anesthetics in Britain and probably in the world. Certainly he wrote the discipline’s foundational textbook. It was he who administered chloroform to Queen Victoria during childbirth.

But his great achievements in anesthetics were not his greatest achievement. He was responsible for the greatest single feat of epidemiology ever. It would hardly be too much to say that he founded the discipline.

Perhaps it was his knowledge of the diffusion of gases that first led him to question the principal theory of cholera epidemics at the time—that they resulted from a gaseous miasma that emanated from some putrid source of decay (of which, of course, Victorian London had no shortage). Any infective gas would become extremely dilute at a short distance from the putative source, he reasoned. The distribution of cases in the 1854 outbreak—or any other outbreak in fact—was not compatible with the gaseous diffusion of the disease.

Snow also took the common view that a disease agent whose principal pathological effect was on the gastrointestinal system probably had been ingested, not inhaled. This was not decisive evidence, of course, but Snow was nonetheless right.

By examining the distribution of cases, starting from the new hypothesis that cholera was waterborne rather than miasmatic, Snow deduced that the epidemic’s source was contaminated water from the Broad Street pump, and he persuaded the reluctant authorities to remove the pump handle so that water could no longer flow. A local clergyman, the Reverend Henry Whitehead, at one time skeptical of Snow’s theory, then proved that the initial patient who began the epidemic (the index case, as epidemiologists call it) had poisoned the pump’s water supply.

It took a while before everyone accepted the waterborne nature of cholera, though the theory did win over the leading medical statistician of the day, William Farr, who initially believed that it was elevation above sea level that accounted for the distribution of cases, with filthy, cholera-carrying air stifling lower elevations. One of the greatest engineering projects of all time, the construction of a proper sewage system and clean water supply for London, then got underway, and no large-scale waterborne epidemics have plagued the city since. Indirectly, Snow made vast urban agglomerations safe, for the developed world soon copied London’s example.

This book is a recounting of the Broad Street pump episode. Well written and jargon-free, it will introduce an extremely significant historical event to a wide readership, who might otherwise know nothing of it. And this is important, since we should not take for granted all the advantages that we enjoy—clean water and an absence of waterborne epidemics, for example—as if they were not the product of great intellectual courage and practical intelligence in wresting knowledge from ignorance: for if we are grateful for nothing, we will find ourselves prey to the inevitable dissatisfactions of life and think ourselves the most unfortunate of creatures.

For the most part, Johnson’s prose is good and plain, though occasionally he lapses into regrettable journalese. When he writes that without the genes that a virus contributes to the cholera bacterium, the germ “literally doesn’t know how to be a pathogen,” and that “it learns to be a killer by borrowing genetic information from an entirely different species,” we have entered the realm of anthropomorphic over-dramatization. Such blemishes, however, are few.

In reflecting on the episode, Johnson treats modern epidemiology as an unequivocal benefit to mankind. So, grosso modo, it is. But there is one slight drawback: the latest unconfirmed findings (of which there are many every month) that correlate some environmental factor or another with a dread disease (coffee, say, with pancreatic cancer), create an atmosphere of anxiety among the healthy where none is necessary, because the correlations turn out to be insignificant statistical artifacts.

For those who know no medical history, this book will be a treat.


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