Is epidemiology really exactly 350 years old? You may ask: only 350 years? What about Hippocrates, the “father of epidemiology” who lived 2500 years ago? Or you may skeptically observe that epidemiology cannot be that old because John Snow, the “founder of epidemiology,” was born only 200 years ago. You may also wonder whether the origins of any scientific discipline can be captured by a single birthdate. Does physics, biology, or mathematics have a birth date? Is not epidemiology, such as these other sciences, the result of a historical process with well-defined milestones?
I cannot discuss how physics, biology, or mathematics was born, but epidemiology has a characteristic that differentiates it neatly from these other sciences: its mode of knowledge acquisition requires the prior existence of a concept of “population.” Without an understanding that populations are more than aggregates of individuals, the comparison of groups of people makes no sense. And without group comparisons, there is no epidemiology.
Many aspects of human intelligence and medicine’s ambitions have conspired against the emergence of population thinking. First, population thinking is not intuitive: it is not immediately obvious why the study of populations gives access to knowledge that is inaccessible from the study of individuals. This is in contrast with the trial-and-error approaches of Archimedes or Galileo, which make much more intuitive sense. Second, until recently, doctors were not attracted by population thinking. For most of recorded history, they have aspired to understand and treat individual patients on the basis of exact knowledge. To aggregate information from individual patients inevitably means the loss of some distinguishing information because no two persons get sick in exactly the same way for exactly the same reasons.
As a matter of fact, historians have not yet identified a doctor of antiquity or the Middle Ages, in Asia or in Europe, who grouped patients to obtain supraindividual information, even though many of those doctors made eminent contributions to the evolution of the clinical sciences. For 4000 years, from the emergence of physicians until well into the 19th century, all civilized medical systems shared an approach to health and disease that can be characterized as holistic. This “antique” holistic medicine, to differentiate it from its modern revivals, considered health to be the result of a complex equilibrium between the human body and the constitution of a geographical area, that is, almost everything in the universe.1 The symptoms of a patient were viewed as a result of a specific imbalance in this complex equilibrium. Doctors used all they knew about their patients to characterize this imbalance, and eventually to propose ways to correct it. In modern terms, this would be equivalent to explaining a patient’s symptom (eg, abdominal pain) based on innumerable factors (eg, season, phase of the moon, personal temperament, wind direction) and all their multiway interactions. Holistic medicine focused on individuals and was incompatible with epidemiology. This was the case even in its most rational, nonreligious forms, such as Hippocratic medicine.2,3 Hence, at least until the 17th century, medicine did not use a concept of “exposure” (contextual factors were viewed as increasing a person’s susceptibility to certain types of symptoms, but not as direct causes of symptoms) or a concept of “disease” that would stand out as the common diagnosis of a group of patients. Each patient was different.
The term antique holistic medicine is not derogatory. On the contrary, antique holistic doctors aspired to a personalized medicine that we continue to strive for today. It was in fact a humble retreat from these high goals—an acceptance of our ignorance of the exact determinants of individual health—that shook the foundations of the entrenched holistic–individualistic approach and ultimately gave birth to epidemiology.
Historically, epidemiology did not emerge progressively. It existed in 1662, but not in 1661. Epidemiology sprang from a singular event triggered by simultaneous and profound movements away from holism in several spheres of philosophy, science, medicine, and society. These movements were concentrated in a few decades of the 17th century and powerfully converged in 1662, exactly 350 years ago. Let us review the components of this explosive mixture of social and intellectual ferment leading to 1662.
Crisis of the 17th Century and the Plague
The first feature of the 17th century relevant to the emergence of epidemiology is the availability of population data about causes of death, in particular deaths from plague. The 17th century was an especially troubled time in Europe.4 A severe economic recession had produced famines, misery, sicknesses, and epidemics. There were terrible, complicated wars, including the Thirty Years’ War. There were many social and political revolutions. Most failed, yet the Spanish Empire collapsed, and, during the English Civil War, King Charles I was beheaded. To reign in political instability, states began to assume political and military prerogatives that had previously belonged to local powers. However, these emerging absolutist states faced a major obstacle: plague.
After the Black Death of 1348 to 1350, outbreaks of plague recurred in Europe with increasingly tight periodicity between the 14th and the 17th centuries.5 These outbreaks became extremely disruptive to both political and economic life. Wealthy people, politicians, magistrates, and even doctors fled cities with each return of the plague. Soldiers were likewise securely sheltered in the countryside. Administrative and political institutions were paralyzed, with no authorities to quell rioting and looting.
The kings of England attempted to reduce the chaos that accompanied each outbreak of plague by monitoring the plague’s progress. The Church of England was mandated to install a surveillance system for plague deaths at the level of local parishes. These first health-related population data were the famous London Bills of Mortality.6 Elderly women, sometimes assisted by surgeons, identified cases by investigating the houses where deaths had occurred.7 This information was routed by the local parish clerk to the clerk of London, where it was printed and sold on a weekly basis. Thus, when those who were financially able to move out of London read in the Bills that plague deaths had seized the poor parishes, they could organize an ordered retreat, hopefully limiting the extent of the mayhem.
The Bills of Mortality played a decisive role in the birth of epidemiology because they became available for analysis as a series of health data collected in a relatively similar way, on a weekly basis, over several decades since 1603.
New Ideas in Philosophy, Science, and Medicine
The second relevant feature of the 17th century was a simultaneous blossoming of fresh ideas in philosophy, science, and medicine, leading to new concepts of “exposure” and of “disease,” and a drive away from the complexity of holistic thinking. The new worldview was compatible with population thinking and group comparisons.
A modern concept of exposure was formulated by Francis Bacon. In his health research agenda designed to prolong human life, he encouraged investigating the role of determinants that epidemiologists still study today: height and weight, date of birth, food, diet, behavior, exercise, housing, heredity, and medical treatments.8
Thomas Sydenham,9 (p.92) meanwhile, made a decisive contribution to a concept of outcome. In the signs and symptoms shared by different sick people, he saw the manifestation of generic “diseases” with characteristic natural histories. Patients with the same disease could then be grouped and compared.
In addition, the drive toward simplification was best expressed by René Descartes in 1646.10 He proposed an approach to science and knowledge, now known as reductionism, that abruptly parted ways with the complexity of ancient holism. Descartes’ central idea was to simplify working hypotheses, consider one factor at a time, ignore interactions, and rely on evidence rather than speculations.
In the holistic approach, a multiplicity of interacting causes divided populations into individuals. In the reductionist approach, individuals could be grouped according to a shared disease or a shared exposure, and then compared. Reductionism focused on one exposure and one outcome, as well as, eventually, sets of confounders.
Today, the qualifier “holistic” has positive connotations whereas the qualifier “reductionist” does not, especially in the context of medicine. Holistic medicine considers the complaints of the “entire” (holos in Greek) person, whereas reductionist medicine focuses on a symptom or on a diseased organ. However, medicine was holistic for 4000 years before it became reductionist. Reductionism was the indispensable step toward the implementation of group comparisons and, with it, the emergence of epidemiology.
Observations on the Bills of Mortality
The availability of population data and the development of a reductionist approach, combining Baconian and Cartesian ideas, created the conditions for the crucial step: the application of population thinking to health data.
John Graunt, a businessman admitted to the Royal Society, published a book in 1662 entitled Natural and Political Observations Made Upon the Bills of Mortality. In it, Graunt11 summarized the analyses of 50 years of data extracted from the Bills of Mortality. The “political observations” mentioned in the title dealt with questions such as the size of the London population and the appropriateness of monogamy. The “natural observations” involved mortality from various causes and from the plague, in particular.
In his analysis of the Bills, Graunt literally had to start from scratch. To make sense of the 50 years of weekly bills, maybe 2500 sheets originating from “several great confused Volumes,” he produced the first known tables of health data.6 An example is shown in Figure 1. Figure 2 is a plot of some of Graunt’s tabulated data. The shark-toothed evolution of plague deaths is quickly apparent to our modern minds. There were years with plague and others without. Some outbreaks were horrendous. In 1625 alone, Graunt computed that, after correction for misclassified plague deaths, the plague had killed 46,000 people—about 38,000 more than all other causes of death together.
However, this irregularity of plague outbreaks was what Graunt would have expected. He knew by experience that the plague had erratic behavior. What Graunt must have found much more surprising is a phenomenon that could not have been observed previously: the regularity of mortality from causes other than plague.
Imagine you are John Graunt. What would be your perception of the occurrence of death if you had never seen mortality statistics? Death on an individual level is haphazard and unpredictable. Why would death be regular and predictable on a population level? Even today, this question has no simple answer. Graunt may not have been able to explain it, but he was clearly fascinated by the predictable trends observed in his population data. He wrote,
Among the several Casualties some bear a constant proportion unto the whole number of Burials; such are Chronical diseases, and the diseases, whereunto the City is most subject.11 (II:19)
The Chronical Diseases shew the ordinary temper of the Place, so that upon the proportion of Chronical Diseases seems to hang the judgment of the fitness of the Country for long Life.11 (II:15)
The terminology is Baconian, but we are witnessing here the first report of observations derived from population health data. Graunt realized that some knowledge can be acquired only at the population level. Regularity of the causes of death during periods when London was free of epidemics provided a stable, “expected” number of deaths that he could then compare with the number observed during plague years. These comparisons also allowed him to identify outbreaks of specific causes of deaths, check the quality of the parish reports, and detect misclassifications of causes of death. Suddenly, Graunt had access to a feast of new information with which he jubilantly addressed an array of political and natural questions that had up to that time been unanswerable. This explains why, in Kenneth Rothman’s words, “with this book Graunt added more to human knowledge than most of us can reasonably aspire to in a full career.”12
Graunt’s analyses gave him a clue to the origin of the plague. He observed that the irregularity of death rates distinguished the plague from the “chronical” causes of death. In 17th-century London, tuberculosis killed about 2000 people a year. Plague was different, it took 46,000 lives in 1625 and zero 4 years later. For Graunt, the contrast between the irregularity of plague deaths, which he described using deaths per week, and the regularity of the deaths from chronic diseases suggested that plague had an environmental origin:
[T]he Contagion of the Plague depends more upon the Disposition of the Air, then upon the Effluvia from the Bodies of Men. Which also we prove by the sudden jumps, which the Plague hath made, leaping in one Week from 118 to 927: and back again from 993 to 258: and from thence again the very next Week to 852. The which effects must surely be rather attributed to change of the Air, then of the Constitution of Mens bodies. 11 (IV:11–12)
In Graunt’s time, the Black Death was thought to have been provoked by a specific alignment of planets, and then transmitted by miasms (fetid gases) that emanated from the bodies of the sick and contaminated susceptible people who inhaled them. Hence, the habit of some doctors to fill long leather beaks with dry flowers and odoriferous plants, and place them under their nose when they attended plague patients (Figure 3). Graunt’s analysis pointed to an environmental and fluctuating cause for the plague. Plague outbreaks were not synchronized with astrological (eg, specific planet alignments) or political (eg, coronations of kings) events.
Impact of Graunt’s Observations
Graunt’s book may have been a giant leap for epidemiology, but it was a much smaller step for society as a whole, at least in the short run. This work did not affect the practice of medicine or abolish beliefs in astrology. There are nonetheless several reasons to believe that this publication did not go unnoticed. First, the book sold well enough to justify several publications in the 17th century. Second, written in the aftermath of the Restoration of the English monarchy, the book was explicitly intended to help the government better manage societal and health-related questions. His conclusion was the following:
That a clear knowledge of all these particulars, and many more, whereat I have shot but at rovers, is necessary in order to good, certain, and easie Government, and even to balance Parties, and factions both in Church and State. But whether the knowledge thereof be necessary to many, or fit for others, then the Sovereign, and his chief Ministers, I leave to consideration.11 (last paragraph of The Conclusion)
Third, by a puzzling coincidence, the last great epidemic of London took place in 1665. Could the subsequent disappearance of the plague in any way be related to the publication of Graunt’s book? For example, did Graunt’s quantitative demonstration that plague outbreaks were apparently related to a fluctuating environmental factor stimulate public authorities to more rigorously control the traffic of boats and people coming from plague-ridden areas? Some historians have stressed the role of quarantine and cordons sanitaires after 1662 in the withdrawal of the plague from Europe,13 but other theories involve a change in the rat population (rat fleas transmit the plague) or the replacement of wooden houses by brick ones, which more effectively separated humans from rats.5,14
Whatever its immediate consequences, the 1662 publication by Graunt combined population thinking and comparisons of population data across time, in an unprecedented fashion, and one that bears the unmistakable hallmarks of epidemiologic thinking. The book was the culmination of an extraordinary convergence of historical events, including strong political will; new ideas in philosophy, science, and medicine; and the availability of population health data that were necessary for epidemiology to emerge. A line of thought that had been literally inconceivable became suddenly compelling. This was 350 years ago.
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