BOOK REVIEW
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.
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.
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.
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).