What was the infectious agent of the Black Death that struck Europe in 1348 and succeeding decades? The classical answer is Yersinia pestis, today’s bubonic plague. But if the disease had been bubonic plague, then outbreaks in the human population should have been preceded by extensive deaths among local rodents. The fleas that transmitted the infection among rats would then have been forced to abandon the cold bodies of their former rodent hosts and would have settled on the people who fed and sheltered the rats. But no contemporary account mentions a lethal outbreak of mortal disease (or epizootic) among rodents during the fourteenth or fifteenth century. On these grounds, more than one historian has challenged the identification of the Black Death as bubonic plague. Still, the absence of evidence is not evidence of absence. Sometimes plague is transmitted from person to person by airborne droplets, and an initial epizootic among rodents could have passed unrecorded.

In his short, sweeping, and brilliant book1 the late historian David Herlihy brings new information from a most unlikely source to bear on the identification of the infectious agent of the Black Death. In the aftermath of the plague, the Church received many petitions to canonize persons who were said to have miraculously cured cases of the plague. In trials conducted and carefully documented by the Church, petitioners described the medical afflictions that were alleged to have been miraculously cured. The proceedings of many of these trials were collected in Acta Sanctorum, a seventy-volume hagiographic collection published in Antwerp starting in 1643.2 While some depositions mentioned the buboes, or characteristic lymph node swellings of bubonic plague, Herlihy points out that the “sign of the plague” frequently referred to in these trials is described as petechiae, or spots, characteristic of anthrax and some other diseases (but rare with bubonic plague).

Whether the symptoms of people who were miraculously cured should be taken as representative of the symptoms of the people who died of the plague—whatever it was—is not a question that Herlihy addresses. Miraculous survivals may have selectively favored people not in fact infected by fatal diseases.

Herlihy does not consider the historical evidence so far assembled to be decisive, and I agree. He suggests that rare mutant forms of plague are also consistent with the evidence. The example of HIV suggests to Herlihy that new infectious agents can appear apparently out of nowhere and can also disappear, leaving open the possibility that the agent of the Black Death may not be known to us now.

In October 1998, French scientists published new evidence that the infectious agent of the Black Death was indeed today’s bubonic plague, at least from the sixteenth century onward.3 Urban construction projects in southern France had recently uncovered two mass graves known from historical evidence to contain victims of quarantine hospitals for what was then called “plague.” To avoid possible contamination by any other source, the French team pulled teeth that had never emerged from the jaws of these “plague” victims and extracted the DNA in the dental pulp within these teeth. Seven of the twelve teeth tested showed DNA that matched the DNA of the modern bubonic plague bacillus. When the scientists applied the same procedures to teeth similarly extracted from a medieval grave in Toulon, France, where there was no historical evidence of plague, none of the teeth revealed plague DNA. The case that the “plague” victims were infected with Yersinia pestis is strong.

These techniques should now be applied to plague victims of the fourteenth century. According to Herlihy, many well-to-do people about to die from the Black Death willed that their remains be interred in the floor or walls of their favorite church. If several such crypts could be located and opened, it might be possible to identify the genome of the infectious agent by DNA analysis. The French team suggested that five of the dozen teeth they tested from plague victims may not have revealed bubonic plague DNA because the marker sequences had been degraded. The risk of DNA degradation would be even higher for remains from the fourteenth century.

Whatever the infectious agent may have been, the epidemics, famines, and civil disruptions that began in 1348 and continued into the fifteenth century may, according to Herlihy, have reduced the population of Europe by two thirds between 1320 and 1420. Local reports demonstrated that the populations of some cities and villages in England and Italy fell by 70 or 80 percent in the late decades of the fourteenth century.

Herlihy argues that this catastrophe shook European society loose from a stable condition of high population density, intensive grain production, and widespread poverty. Europe otherwise might have remained stagnant indefinitely, but the plague “broke the Malthusian deadlock that medieval growth had created.” It “assured that the Middle Ages would be the middle, not the final, phase in Western development.” A scarcity of workers following the drop in population created incentives for labor-saving technology, as the survivors among the poor insisted on higher wages. Guilds to which admission had previously been hereditary or strictly limited were forced to recruit more widely, from among the poor. Herlihy’s assessment of the beneficial economic impact of the Black Death essentially repeats the 1973 analysis of Douglass C. North and Robert Paul Thomas.4


Both before and after the catastrophe, according to Herlihy, the demographic pattern in Europe consisted of two tiers: the poor on the one hand and the middle and upper classes on the other. But the Black Death changed the proportions of the population in these two tiers dramatically. In the bottom tier, the numbers of the poor were controlled by what Malthus later called “positive checks,” largely plague and famine. In households with the smallest amount of land (which can be taken as indicating income or wealth), there was no correlation between the size of the land owned and the number of people in the household. In the upper tier, the middle and well-to-do classes regulated their numbers by what Malthus called “preventive checks,” that is, by deferring marriage until a profession or productive assets were available. The upper tier limited reproduction in proportion to available income.

Whereas all but a small part of the population were in the bottom tier before the Black Death, afterward many more people had access to farms and paying jobs. “For a significantly larger part of society,” Herlihy writes, “the care of property and the defense of living standards were tightly joined with decisions to marry and to reproduce…. Out of the havoc of plague, Europe adopted what can well be called the modern Western mode of demographic behavior.”

The broader cultural and political implications of these changes were no less significant, Herlihy argues. The need for many more educated priests to administer the last rites to those who were dying coincided with many deaths among the priests. Many bequests from wealthy people who had died made possible the creation of new national universities. These newly founded schools weakened the monopoly on education previously held by the ancient universities of Bologna and Paris. Because not enough teachers in these new universities knew Latin, the use of vernacular languages spread. The scholars developed national, rather than Catholic, loyalties.

Herlihy suggests that the rise of independent universities may have contributed to the rise of nationalism centuries later. The increased demand for books and scribes stimulated the invention of printing. Conflicts between the mass of poor Christians and the highly educated leaders of the Church, which looked on the proliferating cults of miracle-working saints as superstitious, challenged the monopoly on religious power of the established authorities, eventually opening the way to religious pluralism. Thus the Black Death brought about enduring transformations in European technology, demography, politics, and religion.

Not all historians would accept Herlihy’s vision of the plague as the prime mover of Europe’s transformation. Even among economic historians, there is no unanimity about the underlying economic story told by North and Thomas, which supports Herlihy’s view of social and cultural change. For Carlo M. Cipolla, formerly professor of economic history at the University of California, Berkeley, and now at the University of Pavia, an

urban revolution of the eleventh and twelfth centuries ushered in a new era with the introduction of public schools, and many a city witnessed a noticeable development of elementary education…. The town was to the people of Europe from the eleventh to the thirteenth centuries what America was to Europeans in the nineteenth century. The town was the “frontier,” a new and dynamic world where people felt they could break their ties with the past, where people hoped they would find opportunities for economic and social advancement, and where there would be ample reward for initiative, daring, and hard work.

In Cipolla’s view, the plague was important for what it permitted, not for what it directly caused. Because it killed so many Europeans, it assured that “Medieval and Renaissance Europe did not go the way of Asia. European development was not halted by the suffocating pressure of population.”

Though DNA tests can, with luck, identify the infectious agent of the plague, historians cannot yet discriminate decisively between Herlihy’s image of change and Cipolla’s. As Cipolla writes,

Easy explanations of complex historic phenomena charm people, precisely because they are easy and, therefore, reassuring. The explanation pleases, the problem irritates. And yet the explanation is often unattainable, while the problem remains the only valid thing.5

Was the Black Death a product of the situation in Europe, or was it an unpredictable, external shock? A Florentine chronicler, Giovanni Villani, who died of the pestilence in 1348, argued that the plague was divine punishment for the current sins of the Florentines: “avarice, greed and usurious oppression of the poor.” Earlier in Herlihy’s career, as Samuel Cohn points out in his introduction, Herlihy agreed with Villani that the plague arose out of particular social attitudes and practices. But by the time Herlihy gave the 1985 lectures on which this book is based, he had changed his mind. As mentioned earlier, one of several reasons was the rapid spread of AIDS, which appeared to him to have come unexpectedly, out of nowhere. In shifting his position, Herlihy illustrates how large events today illuminate and change our understanding of the past.6


Conversely, Herlihy’s analysis of the past suggests the potential consequences of demographic and epidemiological changes taking place today. At the end of the twentieth century, the growth rate of the global human population is dropping rapidly by historical standards (though not rapidly enough in poor countries to avert much preventable misery). During the twenty-first century, this growth rate may become negative and absolute population size may decline, as it is already declining in some European countries.

Fourteenth- and fifteenth-century Europe provides one of the few documented examples of how societies responded to widespread—not merely local—decline in population. A dramatic fall in the abundance of people was followed by an increase in their value. Parents, as Cohn points out, shifted much of their bequests from pietistic charity to their children. Increased land per person shifted diets toward more meat (the food of the rich previously). The scarcity of people raised the wages of both agricultural and urban laborers and stimulated the development of technology. From an oversimplified economic perspective, when the supply of people dropped, the price of people rose.

By contrast, if the decimation of the Amerindians following the European conquests raised the price of people in the New World, it also led Europeans to tighten their control of the subjugated populations. This example shows that the effects on well-being of a major drop in population depend as much on the relations of power in a society as on numbers of people.7

In the twentieth century, the supply of people has surged to unprecedented levels. The absolute number of people has nearly quadrupled, from perhaps 1.6 billion at the beginning of the century to just over 6 billion expected by its end. Since World War II, the growth rate of global population has been, and remains, higher than ever before in history. To judge by the preventable ills of the human population today, people collectively are valued cheaply. Three quarters of a billion people are chronically undernourished; at least another billion are malnourished; a billion adults are illiterate; perhaps two billion people are infected with the tuberculosis bacillus (with hundreds of millions more under threat from other infectious diseases); and roughly four fifths of the world’s population live on average annual incomes of approximately $1,100.

Though little confidence should be attached to predictions of the trajectory of global population more than a very few years into the future, a quadrupling of human population size (to 24 or 25 billion) will probably not recur in the coming century. Population size could peak and even begin to decline within the next half-century. The transition from a doubling of population in the last forty years of the twentieth century to a possible absolute decline in the twenty-first century could be accompanied by a rise in the value of people, other things being equal, although such a hypothesis is of course speculative.

In 1949, the British historian and sociologist T.H. Marshall identified three elements of citizenship: the civil, the political, and the social.8 The civil element consists of individual liberties: freedom of speech and religion; the right to property, contracts, and justice. The political element consists of the right to exercise political power as a voter and public official. The social element, for Marshall, includes “the whole range from the right to a modicum of economic welfare and security to the right to share to the full in the social heritage and to live the life of a civilized being according to the standards prevailing in the society.” Marshall suggested that the formative period of civil rights occurred in the eighteenth century, of political rights in the nineteenth, and of social rights in the twentieth. The formative periods Marshall named coincide with the modern period of rising population, and with the epidemiological transition from infectious to chronic diseases, beginning in the eighteenth century and continuing until now. He spoke just at the start of the largest and fastest global increase of population ever to take place.

The analogy with the fourteenth and fifteenth centuries suggests, all other factors being equal, that the civil, political, and social elements of citizenship would have diffused more rapidly had population growth been slower after the eighteenth century. Nearly half a century after Marshall spoke, it seems clear that the formative period of social rights is not yet over. If the anticipated slowing or reversal of the growth of the human population in the twenty-first century occurs, it could be expected to increase the incentive to nurture well those who are born, and it could speed the worldwide diffusion of these desirable elements of citizenship.

When it can no longer so easily be assumed that there will be plenty more people to come, then assuring that people have the political and economic capacities sufficient for food, education, health, and a meaningful civic life may take on greater urgency. But as the example of the Amerindians shows, this positive outcome is by no means inevitable. If major changes for the better do occur, it will be to the lasting credit of human beings that, this time, the demographic changes were brought about in large part by the reproductive choices of individuals, rather than by the catastrophe of the Black Death.

This Issue

March 4, 1999