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Two Tales of a City

Christopher Hamlin

The Ghost Map: The Study of London's Most Terrifying Epidemic—and How It Changed Science, Cities, and the Modern World. Steven Johnson. xviii + 299 pp. Riverhead Books, 2006. $26.95.

The Strange Case of the Broad Street Pump: John Snow and the Mystery of Cholera. Sandra Hempel. xii + 321 pp. University of California Press, 2006. $24.95.

To epidemiologists, the London doctor John Snow (1813-1858) is no mere pioneer—he is an icon for the discipline, whose still-cited work represents a common core of method and rigor. In the treatise for which he is famous, On the Mode of Communication of Cholera (1855), Snow elucidated the means by which the disease was spread during the London epidemics of 1848-1849 and of 1853-1854: through fecal-oral transmission of a specific pathogenic agent in contaminated water. He reached this conclusion chiefly on the basis of two natural experiments.

John SnowClick to Enlarge Image

First was the investigation Snow made in the summer of 1854 of an area of south London served by two water companies, one using an upstream source, the other drawing from the sewage-ridden tidal Thames. Because these rival companies had at one point competed head to head, some streets had beneath them mains from both companies, with adjacent homes relying on one or the other for service. Such conditions permitted something like an accidental randomization of every variable except water source. But Snow found profound differences between the two companies (nearly an order of magnitude, he claimed) in the number of cholera deaths per household served.

Better known is Snow's mapping of cases of cholera in Soho near the Broad Street pump, a hand-operated affair that served up drinking water from a shallow well. There Snow focused on a sudden eruption of cholera within a single densely populated neighborhood. He showed that use of water from the Broad Street pump was a common factor in almost all of the cholera deaths and also that nonuse of that water was a characteristic of two groups (workhouse residents and brewery workers) that suffered little from the disease. In likening the behavior of the apparent cholera agent to a living thing, Snow is often listed as a pioneer of the germ theory. Empirically, he predicted the characteristics of Vibrio cholerae, the organism that Robert Koch would identify almost three decades later (and which Filippo Pacini had described much earlier, at about the same time that Snow was carrying out his investigations).

In sum, Snow is often presented not simply as an innovator (or exemplary user) of epidemiological methods, but as a paragon of scientific medicine. In her book The Strange Case of the Broad Street Pump, Sandra Hempel, paraphrasing a Victorian enthusiast, describes Snow as follows:

What distinguished Snow . . . was his tireless determination to pursue every scientific investigation relentlessly to its logical conclusion. There were no short-cuts, no leaps of faith and no unquestioning acceptance of untested traditional wisdom, but organised procedure, sound experiment and careful observation. This step-by-step approach, which seems to have come naturally to Snow, was in fact a precursor of the modern method that is now the basis of all medical research.

Snow's personal characteristics are usually lauded too: he was saint as well as scientist, gentle and unambitious, a vegetarian in an age of beef and competition.

Most now accept that Snow was correct about the cause of cholera and that his demonstrations should be regarded as conclusive. Yet his colleagues were unpersuaded. His hard work never translated into the deserved rewards of prestige and position, and many Snow scholars, starting with his friend and memoirist Benjamin Ward Richardson, have sought to right that wrong. Their subject stands as a lonely and righteous victim of the pettiness and stupidity, and occasionally the venality, of medicine and public health in that era.

Cartoon by George John PinwellClick to Enlarge Image

Journalists Sandra Hempel and Steven Johnson have recognized how well Snow's story lends itself to a particular genre of mass-market history of science. The form is as follows: a protagonist, an outsider representing truth and virtue (qualities that are linked through some unexplained dynamic of reciprocity), takes on entrenched intolerance. Hempel explains that she came to Snow in search of a subject that would "combine science with colour and human interest" and would have "a strong narrative . . . with twists and turns and cliff-hangers and finally, a satisfying conclusion." The two books have titles that suggest they are a "mystery" and a "ghost" story, respectively (the ghosts being those who died in "London's Most Terrifying Epidemic").

Both authors present the familiar Snow, with a large dose of "you are there; feel your griping guts"—an element usually absent from academic writing. Each goes beyond Snow to include the supplementary inquiries of Rev. Henry Whitehead, whose parish included Broad Street, and of the parish's own investigating committee. Hempel and Johnson also discuss the east London cholera inquiries of 1866, which they see as marking adoption of Snow's views by the English public-health establishment (but here they overstate their case, in my view).

There are few surviving sources that shed direct light on the evolution of Snow's thinking. As storytellers, these authors speculate more boldly than would most historians (sometimes warning the reader when they are doing so), but if much is conjectural, nothing is implausible. Both draw on an impressive but narrow array of published and archival sources relating chiefly to Snow and Soho, but they appear to have sampled only a small amount of the relevant scholarship in epidemiology and medical history. Much of the authority of these two books derives from the magisterial Cholera, Chloroform and the Science of Medicine: A Life of John Snow (Oxford 2003), by Peter Vinten-Johansen and others, which is the product of many years' work by a team of researchers representing medical history and the several specialties of medicine in which John Snow worked. (Johnson is more clearly dependent on this source than is Hempel, who used it only as a check.)

The most visible influences on Hempel and Johnson, however, are the novels of Charles Dickens. The Ghost Map and The Strange Case of the Broad Street Pump depict a world of delightfully eccentric stick figures, going through predictable motions on a landscape riven by an obvious moral divide. Hempel's "boy's own" John Snow is a Dickensian hero—modest, stolid, dogged, even a bit of a "prig," she admits. For her, he is the last of a long line of English doctor-generals, starting with William Reid Clanny in Sunderland in 1831, who have come forth to give battle to cholera. Johnson gives us a similar portrait of Snow but folds into the text minor characters and subplots that seem to come straight from Bleak House—the enigmatic scavenger who harvests the city's filth, the utter victimization of poor people (such as street-sweeper Jo, source of a chapter title) and the sheer lack of accountability of public institutions.

If Hempel's book is a saga of campaigns against cholera from 1831 to 1866, Johnson's is a screenplay. He zooms out from Snow and Whitehead to show us the giant city from above, and then takes us into a cracked cesspool, giving us a vibrio's-eye view. The approach works well, except for the digressive sermons, built of mixed metaphors, that punctuate and conclude the book; the last 50 or so pages of The Ghost Map discuss the benefits and vulnerabilities of present and future cities, but in ways that take us far from Snow and epidemiology.

Both writers wish to tell moral tales, as is appropriate. Snow's achievement should interest us. We too face grave threats, many arising out of the combination of evolving microbes and an enormous and concentrated host population. But neither Hempel nor Johnson can say quite what the moral should be. It seems to be something like this: "If you are going to tackle a problem, get the right answer," and evidently this will happen more or less automatically if you are virtuous and industrious. Here the influence of Dickens—whose answer to the problems of his day usually reduces to the maxim "Be Kind"—is plain.

Snow's map of cholera epidemicClick to Enlarge Image

The failure to explain how Snow is relevant to us reflects a broader cognitive failure, jointly of historical analysis and the representation of epidemiological reasoning. The chief historical fallacy is presentism. Retrospectively, the story is so simple: good versus bad, truth versus error. Our post-Koch conviction that Snow was on the right track makes it seem as if his arguments should have been enough for his contemporaries too, had they only been honest. Both authors struggle to label those who disagreed with Snow. Johnson is usually blunt, allying the miasma theorists with "folklore and superstition." His judgment that the statistician William Farr, who associated cholera with elevation, was "brilliant" is undercut by his reference to Farr being "drawn into error by orthodoxy and prejudice." Hempel tries to apologize—Victorian doctors had no scans or x-rays—but later she savages Florence Nightingale and Edwin Chadwick as "clinging desperately to their creed" (that the causative agent was in an aerial miasma). Hempel and Johnson perpetuate a tired trope of past medicine as patently and horrifically silly or venal or both. They characterize the therapeutics of Snow's age as errant quackery uninformed by experience or theory. Some of it was. But medical theories were rational (if, in retrospect, partial or erroneous); doctors developed, and shared experiences; and, for many, their commitment to patients or to science cost them their lives. Snow was not the only hero.

Most important, Hempel and Johnson fail to do justice to the complexity of the etiology of the day. They ridicule a "miasmatic" theory—the view that diseases come from a something in the air, related to rotting matter, which usually stank—and Johnson goes so far as to explain adherence to such views as a product of natural selection rather than of induction: People have evolved to want to avoid objects that stink because doing so is a survival mechanism. But there was no single miasmatic theory: Proponents posited a host of ways in which various aspects of atmosphere might account for a number of distinct features of disease. And, for most of these theorists, a causal account was a reconstruction of pathogenic events presumably sufficient to generate the observed effects, one that acknowledged multiple kinds and levels of causes, some of which, like internal constitution and environment, might act in combination. Within this framework, "miasm" and "contagion" were loose terms, and both might be components of an explanation, whether of a single case or of an epidemic. To attribute disease to an internal state ("constitution" or "character") was not necessarily an alternative to recognizing environmental causes, for these might alter constitution or character. Although Snow was generally leery of cause-talk (preferring to refer to the "mode of communication"), he was promulgating a radical simplification in focusing on a single factor.

His contemporaries also had broader expectations of what aspects of an epidemic required explanation than did Snow. At least three questions were usually important to them: the episodic nature of pandemics (which seemed to require a spark, such as a change in an atmospheric variable), the operation of local causes, and a sufficient account of the presence (and course) or absence of disease in each victim and nonvictim, respectively. As a founder of an epidemiology of populations (rather than of individuals), Snow focused only on the operation of local causes.

Thus, what Snow's contemporaries thought he had demonstrated was much less than what he thought he had shown. He had, some admitted, linked cholera to consumption of impure water. It did not follow that cholera invariably or usually arose in this way or that the water was transmitting a specific agent. For two reasons, it seemed better to regard water as only a predisposing agent, more like what we would call a risk factor: First, many, even most, who presumably drank bad water did not get the disease. (At least, this was the case in south London. Snow did not systematically measure unaffected water users in Soho, although Johnson claims that Snow would "intuitively" have understood the importance of such a measurement. Whitehead did try to make such a determination.) Second, since the drinking of bad water had long preceded the epidemic, it could not be its cause. If cholera were contagious, it was unlike smallpox. Why did it not strike every susceptible being?

Other steps, too, in Snow's reasoning were to his contemporaries unacceptably speculative. Johnson and Hempel, following other scholars, suggest that Snow hypothesized oral entry of a causal agent because cholera's manifestations were mainly gastrointestinal. If so, many clinicians of the day would have denied the premise (that the disease was local rather than systemic) and regarded the inference as unwarranted. Medical texts of the day even cautioned young doctors not to be tempted by the seductive assumption, one that underlay the entirety of Snow's presentation, that one should presume like causes for like effects. Epistemic parsimony might be a good rule for philosophers, but it was thought to be a dangerous one when lives were at stake.

Snow could provide reasonable responses to all these objections, but in 1854 they were ad hoc responses. Failure of water drinkers to get cholera may indicate inadequate dose or absence of dose (explicable by positing a particulate rather than a soluble agent) or by what would later be understood as an individual's immunity (in Snow's day, immune status was barely even a placeholder variable). The coming and going of the epidemic could simply be attributed to the coming and going of its hypothetical agent, which seemed not to correlate to any particular regular impurity in water, and might even be present in inverse proportion to more visible evidences of contamination. As a causal assertion, Snow had either stated a truism, that cholera exists when the cause of cholera is present, or he had presented a hypothesis that could not be tested unless the pathogen could be identified. Hence the curious title of his treatise—cautious, with a mix of overstatement and begged question. He had demonstrated a (not the) mode of communication of cholera, but only if one accepted (which many did not) that cholera was explicable primarily in terms of specific agents rather than states of predisposition.

Snow had indicted water. But what was the public to do? He had not shown that contaminated water was invariably deadly (usually it wasn't)—he had shown that it was dangerous. On that, there was general agreement; already Parliament had required metropolitan water companies to improve their intakes, although not all had done so. One of Snow's suggestions—handwashing—now seems dubious: People would be washing in the same water that was deadly to drink, and there was no way to test the abilities of soaps to neutralize that virulence.

In short, Snow's achievement was more complicated—and much richer—than these authors represent. He did not merely demonstrate cause but changed what cause meant. What he began was less the replacement of a worse theory with a better one (water not air) than a paradigm transformation of explanatory expectations.

In retrospect, to disentangle the several causal questions in a way that allowed some of them—those concerning necessary agents—to be studied experimentally was an important way forward. Hempel and Johnson ridicule the approach of the English government's team of scientific inquirers to the Soho outbreak: to gather information on "anything and everything," complains Hempel; "folly," "comic," asserts Johnson. But the approach of those investigators resembles much modern epidemiology. Although their choice of variables to study was hardly theory-free, their instincts were for a careful empirical study that would move beyond the plausible correlations so plentiful in the cholera literature of the day. One can see them groping toward a multivariate analysis long before the necessary mathematics, not to mention computing power, was available.

Thus, where Snow excelled was in what Peter Medawar called the "art of the soluble." Snow saw a way to extract a workable program of research out of the complexity of phenomena. Later, the excluded questions would be reintroduced, and to a remarkable degree we can now integrate the ecology and evolutionary dynamics of microbes with their modes of transmission and the mechanisms of infection and immunity.

Must popularizations of science oversimplify so brazenly? They need not. Medical history has gone beyond Richardson, the memoirist; nor are Dickens's novels the apotheosis of scholarship in British history. Readers have the right to expect authors who write about the history of medicine and the logic of epidemiology to have read reasonably deeply in the recent work in these fields—all the more so if, as is the case with Hempel, one's book is put out by a university press. They also have the right to expect that those who recount a scientific controversy will take the trouble to understand, sympathetically, the arguments on both sides. Here, the mismatch between potboiler writing and the history of science is glaring, for to highlight heroes one must create villains. But science, in Snow's day as in our own, relies on a community of reasoned dissent and respectful criticism.

To those interested in Snow, I recommend either the work of Vinten-Johansen and colleagues, which is both scholarly and readable, or the two central chapters in Charles Edward Amory Winslow's The Conquest of Epidemic Disease (1943; reprinted in 1980).

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