Should We Teach That the Cause of the Black Death Was Bubonic Plague?

Phyllis Pobst. History Compass. Volume 11, Issue 10, October 2013.

For most of the 20th century, historians accepted bubonic plague, Yersinia pestis, as the cause of the 14th‐century Black Death, although a wealth of primary sources seems to conflict with that diagnosis. Scientists focused on Y. pestis have been less certain than historians, arguing over their findings and how these might apply to the 14th century. Historians who believe the diagnosis was premature point to the speed with which the disease travelled, and killed; its symptoms, and its extraordinary morbidity and mortality rates; the shortage of evidence for rats or fleas; and the seasons and climates in which the disease thrived; and they question whether every inconvenient bit of medieval evidence can be covered by later hypothetical mutations. Yet, guided by textbooks, and by popular media that distort the work of scientists, teachers often omit medieval evidence that doesn’t fit Yersinia. Uncertainty over the cause of the Black Death offers historians a chance to show how historical knowledge is generated, by realistic use of primary sources in dialogue with other scholarly disciplines.

My students are a blood‐thirsty lot. They relish horrors and disasters, the disgusting and the violent. So, I imagine I have their full attention when I lecture about the “Great Mortality” of the 14th century—until the questions come. “Why didn’t they just kill all the rats and end it?” And, “didn’t the poor suffer more from the plague because they had more fleas?” Then, I repeat what I have already said: that primary sources for the 14th century Black Death show no evidence of rats or fleas. None. To students, this is a mystery; for teaching historians it is a pedagogical problem, because there is a discontinuity between the primary sources and “common knowledge.”

The Black Death can either be an excellent case study for historians, with which we teach our students the primacy of the primary sources, and show them how to handle these sources with respect and an understanding of their limits, or a stumbling‐block. If we leave the subject to scientists who have little interest in our sources, and less understanding of them, we teach our students that history is a “soft” discipline: mere story‐telling for those who couldn’t make it in the “hard” sciences. Does historical analysis produce knowledge, or not? How textbooks and teachers handle the Black Death may reveal more to our students than we realize.

From teaching about the Black Death to undergraduates and grad students, and from the research required to keep ahead of them, I have come to believe that we historians should be more circumspect about accepting modern diagnoses of that pandemic. We have run after every new scientific pronouncement on the subject, often without knowing enough about the sciences in question to judge the completeness, soundness or relevance of the material. In fact, most teaching historians don’t read the scientific evidence itself; they read articles that distort it in the popular press. My purpose here is not to recommend another diagnosis than Yersinia pestis, but to urge that we remain skeptics until there is more agreement among scientists and sources than we have seen so far. Primary sources don’t match any modern disease, and grave‐robbing scientists don’t agree among themselves. We should not teach what is still unproven as if it were a certainty.

What caused the Black Death? Fourteenth‐century Europeans made it clear that the epidemic disease of the 1340s was new to them. Many writers sketched it in detail for their successors, often before dying of it themselves. From the 1340s on, as the disease returned to European cities at short intervals, generations of physicians wrote careful descriptions of the symptoms and prognoses of their patients. There was no decade when the disease arrived without being seen by many who had seen it before and recognized it; so we have a strong chain‐of‐custody, as criminalists call it, establishing that this same disease existed from the 1340s into the 18th century in Europe. Then, it seemed to disappear, at least from Europe.

Historians should always take the primary sources seriously and teach our students to do the same, but in this particular case, we have sometimes failed. The diagnosis of the 14th‐century pandemic as bubonic plague gained a foothold in academia because early 20th‐century historians trusted scientists more than they trusted their own evidence; real science seemed to be answering the secrets of the universe, and when medieval evidence could not be forced into the bubonic mold, it was ignored.

What Everybody Knows

Students believe they see rats and fleas that are not in the sources because textbooks identify the Black Death as bubonic plague. The connection is confidently stated in many college‐level textbooks and thousands of web sites, often crediting Philip Ziegler for this knowledge. Ziegler is a retired English diplomat whose appealing account of the 14th‐century pandemic has been the touchstone for non‐specialists since 1969. Not an historian, he based it on the best scholarship available in English, and his lively survey pushed most other accounts off the shelf.

Before Ziegler, the most important work in English on the subject was The Great Pestilence (1893) by the magisterial F. A. Gasquet, who adopted the bubonic plague diagnosis when it was brand new. In the second edition, Gasquet noted that, thanks to health authorities in India, much more was known about “the great epidemic of the fourteenth century, now commonly known as ‘The Black Death,’…. Gasquet had considerable influence, and other historians also quickly embraced this idea as fact.

Ziegler’s overview of the state of research in 1969 is the overwhelming favourite of the English‐reading public. It accepts that modern science firmly identifies Pasteurella pestis (Yersinia pestis) as the Black Death. Countless books, encyclopedia articles and web pages repeat his views. The monographs that have appeared recently have been unable to bump Ziegler from his throne. So for college students, this is common knowledge.

A Disagreement of Scholars

A number of respected scholars of the 14th century have challenged the identification of the Black Death as bubonic plague. Since the 1980s, medievalists have looked again at their primary sources and asked whether they confirm—or even allow—a diagnosis of Yersinia pestis. They have not been able to agree among themselves.

Scientists working on the subject also have disagreements with one another. Since the 1990s, improved techniques for isolating microbial DNA and other useful material have drawn teams of scientists into the hunt. Not surprisingly, they have confidence in laboratory tests for answering medical questions, even over many centuries. But what one team of scientists asserts, another team often rejects: the heated dialogue over Y. pestis in tooth pulp is an obvious example. In 1998, a group of molecular biologists in Marseilles claimed to have found septicemic Y. pestis in six teeth from Black Death cemeteries in Provence. In 2000, a team with some of the same members announced that it had found Y. pestis in the teeth of three skeletons from a medieval site in Montpellier. A pair of scholars in the same field responded with an open letter in Science, sharply scolding journals that published studies “that do not observe the necessary controls.” In September 2003, the claim of the Marseilles team was challenged again by the report of two teams collaborating on the original site, and sites in Denmark and England, who concluded that

the absence of Y. pestis‐specific DNA in an exhaustive search using specimens from multiple putative European Black Death burial sites does not allow us to confirm the identification of Y. pestis as the aetiological agent of the Black Death….”

They went on to question the utility of the Marseilles team’s method. A pair of anthropologists pointed out that the first Marseilles report had made several dubious assumptions. First, they had assumed that the graves were from the Black Death period, although the dating was vague. Second, they had used that speculation to assume that the teeth were from Black Death victims, on the surprising assertion that there was a 90% chance that any Marseilles burial from a 30‐year period was a victim of this disease. The anthropologists also suggested that, since the Marseilles lab had used Y. pestis in previous experiments, there was a possibility of contamination of the tooth pulp. A further contribution to the discussion also implied that the Marseilles results were contaminated and urged further testing. Several of the Marseilles team have continued to work on the tooth pulp search and reported in 2007 that they had found more evidence of Y. pestis DNA in teeth, this time from Vienne (the 7th-9th centuries) and Martigues (the 18th century); they argued that they had now proven the usefulness of the method. Interesting as this research promises to be, it’s too early for historians to be certain of the results. Only the participants of this heated debate could believe that it has settled the question of what caused the Black Death of the 14th century.

Historians who don’t have time to make this a priority can easily form a false impression of the state of research on the Black Death, because scientists’ work is often oversimplified by the popular press. There are many examples from the past few years. America Online (AOL) announced in 2008 that the plague, that “medieval scourge,” was returning. The New York Times, Daily Telegraph and others reported in 2010 that the plague (i.e. the Black Death) had been proven to come from China. The BBC confirmed this and equated the plague with the Black Death, repeating all the standard themes. In August 2011, The Guardian told readers that rats “weren’t the carriers of the plague after all,” since the spread of the Black Death was so rapid it could only have been transmitted human‐to‐human. Soon after, the Associated Press distributed a story from Tubingen reporting that the medieval Black Death might be extinct. Nature reported in October 2011 that a study of London skeletons had shown “beyond doubt that Y. pestis was the true cause of the Black Death.” The Nature article was then reported in countless newspapers and online news sites, including academic sites like MEDMED‐L. Such reports of scientific studies appearing in popular media have convinced many non‐specialists that the Y. pestis diagnosis is settled—and by scientists, not historians.

We expect the popular media to oversimplify and stress what is exciting, bloody and terrible; they are selling news as entertainment. But textbooks? Since historians and scientists have not yet agreed, within and among their many disciplines, should textbooks treat the old identification of the Black Death with Y. pestis as a scientific fact? Should students be told, in effect, to find rats and fleas in the primary sources?

What Scientists and Historians Know So Far

Increased scientific interest in the bubonic plague was stimulated by two phenomena: the AIDS epidemic and the collapse of the Soviet Union. AIDS gives scientists greatly increased urgency for understanding how ‘new’ diseases cross from another species, how they are transmitted and how we may control them. The collapse of the Soviet system put a lot of scientific work that had been secret out into the public domain; the West learned that we weren’t alone in experimenting with weaponized forms of diseases. Funding from Western governments for research into Yersinia pestis jumped dramatically after 1991.

So scientists have recently published mountains of data about Y. pestis, most of it in English. Debates over tooth pulp aside, they have established some facts about seven important aspects of the disease. Historians should compare what they have learned with the information from witnesses in the 14th century.

Travel Time

The bubonic plague moves to new regions by slow, gradual steps, since it moves at the pace of a rodent, and they’re home‐loving creatures who remain within small areas. Even with steamships in the 19th century and airliners in the present, the bubonic plague is a plodding disease. But the Black Death was just the opposite. Biologists James Wood, Rebecca Ferrell and Sharon DeWitte‐Aviña pointed out that it “moved from place to place with extraordinary speed… it swept across all of Europe in less than four years.” In fact, no human disease has ever been shown to travel so far, so fast.

The bubonic plague remains in a region for decades, even centuries, without killing off its host population or the human population. It does not disappear, but becomes endemic in host animals, possibly even in the soil. But 14th‐century writers said that their disease spread across Europe at the pace of human traffic, and when it had passed through an area, it was gone until the next epidemic, years later.

Time To Kill

The bubonic plague incubation period is 2-4 days before the onset of symptoms, with an outside range of 1-6 days. Because of this short period, quarantine has no value against the bubonic plague. But 14th‐century witnesses said that, while the disease seemed to kill within a few days of the appearance of symptoms—even in some cases with no outward symptoms—a person could travel a long distance from an infected place without sickening, only to fall ill after being received into a new place. As a result, European cities experimented with periods of isolation, and settled on a quarantine of 40 days, which appeared to be more effective than a 30‐day isolation period. In the 14th century, those without apparent symptoms could incubate the disease for weeks before ‘presenting,’ as the doctors say.

At the same time, there is a wealth of evidence that the medieval disease could kill a new victim within a day of exposure; this is not simply a medieval topos, it is a demographic fact. How this can be reconciled with its ability to travel long distances is not clear, but both features are missing in the modern disease.


The symptoms of bubonic plague are dominated by swollen lymphatic nodules, usually accompanied by a rapidly rising fever; they may also include muscle pain, a stiff neck, chills, nausea, intestinal problems and malaise. If the variant is pneumonic, there is coughing (sometimes of blood), but pneumonic plague is a rare secondary development from bubonic plague. No one dies of Y. pestis without passing days of such symptoms, and there is not much variation within the symptom cluster of each form. The 14th century also reported a range of symptoms, some of which are similar and some different. There might be muscle stiffness, tingling in the skin, a splitting headache, a high fever accompanied by a cough or without it. A person might bleed from the nose or vomit blood. Many witnesses mention a scattering of blood‐blisters like freckles at random across the body that were a surer sign of death than the glandular bubo. A few mention pea‐sized, or egg‐sized, swellings near the lymph nodes. The dying person might give off a terrible stench, and might lie in a stupor for several days before death. But remarkably, witnesses also say that only a few of these symptoms, or none at all, might be present before a person died.


The modern bubonic plague is almost never passed from one human to another. Even in its pneumonic form, it is a disease of rodents, and a human must catch it from a flea that carried it from a rodent. Almost invariably it requires a human being in contact with (the right kind of) flea from a vulnerable rodent, which is in contact with a flea from a resistant rodent: so two fleas and two rodents are needed to infect a human being. While the black rat, Rattus rattus, may be the vector, other rodents can be involved.

Medieval witnesses, on the other hand, observed that the Black Death was passed from one person to another when they were close to each other—relatives embracing, friends clasping hands, priests anointing the sick, wives nursing their husbands. “Breath spread the infection among those speaking together,” wrote Michele da Piazza in 1347, and while this tells us nothing of the actual mechanism of transmission, it confirms the overwhelming testimony that the disease appeared to move from human to human. Medieval witnesses do not mention insects or rodents of any kind. (Animals will be discussed further below.) Some scientists and social scientists acknowledge this awkward evidence, while others dismiss it as unreliable.

Counting the Sick and Dead

In the best documented modern outbreaks of Y. pestis, the morbidity rate was far less than 10%, and the mortality rate among the sick was also usually under 5% (making a very low death rate in the general population). This was true before the invention of antibiotics. A comparison with estimates of medieval morbidity and mortality rates is complicated by occasional extreme statements from traumatized witnesses; this has led some modern readers to reject them all as examples of “the notorious medieval tendency to inflate numbers.” But, as Rosemary Horrox has said, “if the figures are exaggerated, they are not meaningless.” All witnesses agree that the proportion of a city population that fell sick when the Black Death arrived was so high as to be unheard‐of, and that only a few of those who showed symptoms recovered. It is now possible to supplement anecdotal evidence with numerical evidence for some places and groups. Archaeologists have helped with excavations of cemeteries, confirming some extraordinary mortality in densely packed cities like Florence, in numbers that make the 14th century estimates look much closer to the truth than we used to think. An overall loss of between 35-50% of the human population in the pandemic of 1347-53 is now considered conservative.

Rats and Other Animals

Scientists find that the bubonic plague, rather inefficiently, generally requires two fleas and two rodents to infect a human being. But Black Death sources, as mentioned, include no rats or fleas. Historians must beware of dismissing this evidence by making excuses that our forebears were just really dirty and did not notice dead vermin. What iron determination would it have taken to ignore “the sudden death of thousands, even millions, of rats falling from rafters, littering buildings, streets and lanes, not only in 1348, but in numerous other plagues until the nineteenth century”? Yet, no contemporary report of the Black Death mentions rodents in Europe, the Middle East, Asia or Africa. No rats, mice, voles, bats, squirrels or any other rodents appear in eye‐witness accounts of the medieval disease.

The absence of medieval rats is more striking because witnesses listed many other animal except rodents in their narratives of the Great Mortality, including pigs, dogs, cats, domestic fowl and wild birds, horses, cattle and sheep. There are many reports of other species (not rodents) dying in impressive numbers during the Black Death. This does not mean it couldn’t have been bubonic plague—other species can also catch Yersinia pestis—but it makes the absence of rodents more noticeable. Historians must confront the primary evidence in its silence. It is not sufficient to suggest that even the privileged classes of Europe were so inured to the presence of rats that they could fail to see heaps of them dead in the streets and cloisters.

Season and Duration

Y. pestis is a disease chiefly of hot places—India, Africa, the American Southwest—which is most active in those regions in the winter. The Black Death appears to have made no impact in southeast Asia, sub‐Saharan Africa or other regions that now report bubonic plague. Instead, it was as dramatic in northern Europe as in the Mediterranean or the Middle East. While it might arrive at any time, in most parts of Europe, it did its worst in the summer and faded away as the weather cooled.

Witnesses of the 14th century also said that the disease, when it took hold in a town, might burn brightly for three to six months, but did not become endemic. When the bubonic plague appears to go dormant in the summer, it returns again in the cooling autumn; but the Black Death left town after some months, and did not return for years.


These seven points of comparison give historians reason to wait before embracing a diagnosis. The old disease moved as fast and as far as human beings—much faster and further than is normal for rodents. It had an incubation period that may have been weeks, even a month. Victims sometimes died without marked symptoms; most displayed a variety of symptoms that might (or might not) include bubos, fever or coughing blood. The disease appeared to pass from one person to another. It had extremely high morbidity and mortality rates and was intensely contagious. Other animals might die in large numbers in the same period, but no medieval witness (in Europe or elsewhere) mentioned rodents. The medieval disease thrived in the summer, and in cold regions. After some months, it disappeared for years. The modern disease differs in each of those points. The more research is done, the more this is confirmed.

Ockham’s Razor

As scientists examine the molecular structures and genetic history of Yersinia pestis, various mutations have been discovered, and others are suspected. For a disease to jump from one species to another usually requires a mutation. To change from an endemic disease of one species to an epidemic disease in another, an organism must mutate at least once, probably twice (concerning both host and transmission). For an epidemic disease to disappear requires another change. No historian who accepts evolutionary biology will argue that the Black Death was created ex nihilo in the 1330s and miraculously extinguished in the 17th century; mutations are indicated. But to explain the substantial differences between bubonic plague and the 14th‐century epidemic, the number of mutations that must be posited is remarkable—and they must be posited chiefly so that we can hold fast to the diagnosis of Yersinia. This seems to ask urgently for the application of Ockham’s Razor: “plurality must not be posited without necessity.” William of Ockham, who died in the Black Death, frequently invoked this principle of logical reasoning, sometimes called the Law of Economy. The simplest explanation consistent with the known facts, in this case an explanation that does not require a baker’s dozen of hypothetical mutations, should be preferred.

For teaching historians, the Black Death can be an opportunity to show students how archaeology, microbiology, immunology, epidemiology and scores of other disciplines contribute to our understanding of the human past. But we should not treat our own material with contempt, or we will have failed our discipline and our students. Historians are uniquely qualified to assess the value and analyse the content of medieval primary sources and should not allow the glamour of science to make us forget our own expertise. Those who become too fervently attached to a theory must ignore any witness who stubbornly refuses to support that theory, as when Ole Benedictow argues that Boccaccio’s famous description of the symptoms of the Black Death must be wrong, because it does not match the data on Y. pestis. While the testimony of medieval witnesses is sometimes emotional, historians must weigh their credibility on a fair scale. Despising evidence that doesn’t agree with our theories is not the way forward, in science or in history.

This is a time for our usual healthy skepticism, as well as careful research: let’s wait until scientists can agree with each other, in conclusions that can be reconciled with the primary sources, before we teach students that we know what caused the Black Death.