USU 1320: History and Civilization
©Damen, 2019
A Guide To Writing in History and Classics
Index of Chapters
Course Description

Man and Disease: The Black Death

Beginning in 1347 and continuing for a full five years, a devastating plague swept Europe, leaving in its wake more than twenty million people dead. This epidemic now known as the "Black Death" was an outbreak of bubonic plague which had begun somewhere in the heart of Asia and spread westward along trade routes. The consequences to Europe were profound. Besides immeasurable pain and grief, traditional Medieval society was thrown into chaos, economies were fractured, the Church lost status, and art and literature took a turn for the gruesome and bizarre. At the same time, the plague brought benefits as well: modern labor movements, improvements in medicine and a new approach to life. Indeed, much of the Italian Renaissance—even Shakespeare's drama to some extent—is an aftershock of the Black Death. Today its repercussions may be felt in the resistance to AIDS seen in some European populations. By any measure taken, the Black Death was world-shattering and shows how even the smallest of things, the microbial world, can at times steer the course of human civilization.

People, Places, Events and Terms To Know:

High Middle Ages
"Little Ice Age"
Arno River (Florence)
Famine of 1315-1317
Bubonic Plague
Black Death
Yersinia Pestis
Alexandre Yersin
Rat Flea (Xenopsylla Cheopis)

Pneumonic Plague
Silk Road
Boccaccio, The Decameron
Grim Reaper
Dance of Death
Four Horsemen of the Apocalypse

Peasants' Revolt
Brown versus Black Rats
Colin McEvedy
Yersinia Pseudotuberculosis

I. Introduction: Europe before 1347 CE

Europe had experienced a remarkable period of expansion during the High Middle Ages (1050-1300 CE) but that age of growth reached its limit in the later part of the thirteenth century (the late 1200's CE). By then, good farmland had been overworked, and new fields were proving only marginally productive. As the population began to surpass the capacity of the land to feed its inhabitants, famine was imminent.

Worse yet, the climate of Europe was for reasons which are still unclear entering a cooling phase. Whereas in the High Middle Ages a warm, dry climate had predominated, by the turn of the fourteenth century global weather patterns changed for the colder and wetter. Scientists today find evidence of this so-called "Little Ice Age," in polar and Alpine glaciers which the data show began to advance at this time. Moreover, historical records from the day confirm that the winter of 1306-7 was unusually frigid, the first such lingering cold snap Europe had endured in nearly three centuries.

Florence: Arno River (click to see larger image)While the drop in global temperature was probably no more than one degree on average, it was enough to make a significant impact on agriculture. For instance, grain and cereal production had to be abandoned in Scandinavia, and viticulture (wine-production) became impossible in England, as it still is for the most part. Not only cooler but wetter, too, the change in climate brought with it increased rainfall which precipitated other problems, such as flooding. In particular, the Arno River which flows through Florence (central Italy) swept away many bridges with the force of its waters.

But the first real pan-European catastrophe resulting from the onset of the "Little Ice Age" was a widespread failure of crops. Beginning in 1315, the weather was so rainy that most grains sown in the ground suffered root rot, if they geminated at all. Also, the lack of sun, high humidity and cooler temperatures meant water evaporated at a slower rate, which caused salt production to drop. Less salt made it more difficult to preserve meats and that, combined with the losses in agriculture, led to famine by year's end.

When the same happened again in 1316 and then once more in 1317, peasants were forced to eat their seed grain. With little hope of recovery even if weather improved, despair spread across the continent. Frantic to survive, people ate cats, dogs, rats and, according to some historical records, their own children. In places, the announcement of a criminal's execution was seen as an invitation to dinner.

Later branded the Famine of 1315-1317, this disaster marked the beginning of a decrease in European population that would last more than a century and a half. Many cities were hard hit—for instance, in Ypres (Flanders) a tenth of the population died in six months and in Halesowen (England) the population dropped by fifteen percent during this period—all this led to general de-urbanization across the continent.

Nevertheless, these emaciated souls could not have known that worse, far worse, lurked on the horizon. A holocaust of unprecedented fury was stalking them and their children. Out in the hinterland of Asia there was a biological menace massing, a blight that would forever change the face of Europe, the bubonic plague.

II. The Black Death (1347-1352 CE)

The Black Death is the single most significant disease in Western civilization to date, a true and literal plague. The word plague derives from an ancient Greek medical term plêgê meaning "stroke"—it's a reference to the speed with which the disease brings down its victims—and this plague was a real death-blow to medieval Europe. The Black Death, or simply "The Plague," came on its victims so quickly and powerfully and with such a debilitating disruption of facilities it seemed to on-lookers in the day as if the person had been "struck" by some invisible force.

Yet, it was, in fact, not the first time bubonic plague had raised an angry hand to Europe. As far back as 664 CE when it was known as the "Plague of Cadwalader's Time," this disease had swept the continent. But in that age there were far fewer people in Europe and it moved much slower from place to place since there was little trade or travel in the aftermath of Rome's collapse (see Chapter 8). The more well-connected and vital Europe of the years following the High Middle Ages proved a much better host for this plague.

A. The Nature of Bubonic Plague

Devastating as the Black Death was to humankind in the fourteenth century, it is important to remember a central feature of this disease. Normally it does not live among human populations. Plague is endemic—a Greek-based word meaning "(persisting) in a population"—among rodents across the globe, particularly the rats of central Asia where it subsists at a low level and is not widely destructive. When for some reason it breaks out into other biological groups, it can become epidemic ("against a population").

All in all, the bubonic plague is fundamentally a rat disease since it does not persist long in human communities where rats are absent. Rats, however, are not the cause of Plague—its pathogen—rather, just like human hosts, they are victims of the disease. The actual pathogen is a bacillus (a form of bacteria; pl. bacilli) called Yersinia pestis, which was first isolated and identified in 1894 by the French bacteriologist, Alexandre Yersin, after whom it is named. For all the destruction Yersinia pestis left in its wake, people at the time of the Black Death never knew this bacillus was the cause of the Plague. Thus, its invisible mechanisms combined with the extraordinary speed and violence with which it attacked contributed greatly to the terror and psychological damage it wrought upon late Medieval Europe.

Rat Flea: Xenopsylla cheopis (click to see larger image)All the same, knowing the life cycle of Yersinia pestis is essential to the modern understanding of its impact on human history and the course the disease took in the 1300's. This bacillus lives normally as a low-grade infection in the bloodstream of rats. It moves from rat to rat via fleas, in particular, the rat flea (Xenopsylla cheopis),which is in medical terms the vector ("carrier") of Plague. When a rat flea bites an infected rat, it sometimes drinks in Yersinia pestis along with the rat's blood. If so, the bacillus lodges in the flea's digestive tract where it begins to reproduce prodigiously until it forms a solid mass and blocks the flea's digestion.

With its digestive tract obstructed, the flea begins to starve. Frantic from hunger, it hops from rat to rat and repeatedly bites them, but because of the intestinal blockage caused by the clot of bacilli in its gut it can't swallow the blood it's ingested, so it vomits what it drinks back up into the rat's bloodstream. Along with the regurgitated blood come clumps of Yersinia pestis disgorged from the flea's belly. This causes an uninfected rat to become contaminated and, if the rat's immune system is slow to react, the fast-multiplying pathogen overwhelms the animal which dies. But if the rat's immune response is quick, it can counter and suppress the infection. Then, the bacillus continues to exist as a non-fatal parasite living in the rat's bloodstream where it waits until an uninfected flea by chance ingests it. And so the life cycle of Yersinia pestis continues as it volleys back and forth between its two hosts, the rat and flea, using each to infect the other.

Under normal conditions this cycle is restricted to rats and fleas, but if some sort of biological disruption occurs, the disease can spill out of its normal limited niche. For instance, if the rat population declines precipitously for some reason, fleas will be forced to move to other hosts, such as other types of rodents, domestic animals or even humans. While rats are the preferred host of Xenopsylla cheopis, when facing starvation this flea will feed off of almost any mammal.

If infected rat fleas begin biting humans, most of whom do not have resistance to Plague, the disease can reach epidemic levels. In that instance, individuals usually die within five days from the first onset of symptoms, in some cases, overnight. The human immune system is typically overwhelmed by Yersinia pestis which reproduces wildly within the victim's bloodstream. But if it responds quickly enough, survival is possible. If so, the body remembers the infection and pre-empts any second assault. Very few people ever contract Plague twice.

Victim with bursting Buboes (click to see larger image)Because of the terror inspired by this disease and the large number of people afflicted, the progress of bubonic plague as it courses through its victims has been well-documented. Starting with a fever once the immune system has sensed the presence of a foreign organism, the victim's lymph nodes begin to swell as the body tries to flush out the contagion. These nodes are located in the neck, armpits and groin and become visibly enlarged. Called buboes (sing. bubo), swollen lymph nodes are among the most distinctive and painful features of the disease and give it the name "bubonic" plague.

Necrosis of Extremities (click to see larger image)Usually by the third day, the victim experiences high fever, diarrhea and delirium, and black splotches begin to appear on the skin, especially on the tips of the fingers, the nose and anywhere there's a concentration of capillaries. The reason for the black splotches is that the body's smaller blood vessels clog with bacilli and rupture, and blood begins to leak so profusely it becomes visible beneath the epidermis. This is often, though wrongly, said to be the reason the outbreak of Plague in 1347 came to be called the "Black Death," from the darkening of the victim's skin. The "black" in Black Death more likely derives from the Latin word atra, meaning "black, dreadful." Death usually follows soon afterwards, most often from septicemia (blood-poisoning), due to massive internal hemorrhaging as the bloodstream grows congested with bacteria.

This is not, however, the only course the disease is known to take. For example, a victim's buboes can swell so much they burst through the surface of the skin, most often around the fifth day after infection. This process is excessively painful, and Medieval medical records recount how patients seemingly near death would suddenly leap from bed in a frenzy screaming with pain as their buboes burst, spewing out pus and contagion. For all the trauma it causes, the bursting of buboes is, however, not altogether a bad thing. For one, the patient's survival for that long is a good sign in itself—at least half of victims die on average before the buboes have a chance to burst—and the elimination of bacilli through the bursting glands aids somewhat in clearing the infection.

There is worse yet. An even more virulent type of Plague exists which can pass from human to human directly, without employing fleas as vectors. In this form called pneumonic plague, the bacilli are transmitted directly from one human host to another on particulate matter exhaled by the infected. Since the lungs are designed to move air-born material efficiently into the bloodstream, pneumonic plague is especially quick in attacking its victims and almost always fatal. Those who contract pneumonic plague tend to collapse suddenly, cough up blood and die, sometimes within a matter of hours.

There was no cure for bubonic plague in the Middle Ages, none indeed until the discovery of antibiotics in the modern age. In the face of this unknown and irremediable onslaught, Medieval peoples attributed the disease to several factors: "bad airs," witches, astrology and a rare alignment of planets. Its appearance, in fact, brought out the worst in all groups and classes. Moslems blamed Christians, Christians blamed Moslems, and everyone blamed the Jews.

The Black Death was, thus, destructive not only to the physical well-being of Medieval Europe but also its general mental health, a situation which had as much to do with the timing of its onset as anything else. Coming off the peak of the High Middle Ages, people had already been rattled by the disintegration of the Church, the Famine of 1315-1317 and the outbreak of the Hundred Years' War. After the Plague erupted and in just five years killed a quarter to a third of Europe's inhabitants, not only population but morale hit record lows.

B. The Course of the Black Death

There can be little doubt that the Black Death began before the first historical accounts record its presence, but where or how is unclear. Even so, history offers some tantalizing prospects. In researching its origins, it's well to remember a central feature of bubonic plague: it's not at heart a human disease, but one that generally circulates through rat populations. The likelihood is, then, the Black Death began well before 1347 with some sort of disturbance in rodent communities, most likely ones in Central Asia since all historical data point to that as its geographic origin.

Marmot (click to see larger image)As one moves forward in time nearer to the first appearance of Plague in Europe in 1347, the picture becomes better, if still blurry. For some reason, the disease spread on a wide scale to the marmots of central Asia, a mammal resembling a woodchuck or "rockchuck." It's reasonable to assume these animals had little resistance to Plague, causing their population to begin dying quickly en masse. Around the mid-1340's, Asian trappers who hunted marmots for their hides found many dead ones lying around, a seeming boon but with a terrible price tag attached. Ignorant of the danger facing them, the trappers skinned the animals, packed up their hides and sold them off to dealers.

These retailers, then, sent the marmot hides in closed containers down the famous Silk Road, which runs across Asia, all the way from China, through Saray and Astrakhan which are northwest of the Caspian Sea, to Kaffa which is a port on the Crimean peninsula on the northern shore of the Black Sea and at that time was one of the major gateways between East and West. Thus, Plague could not have landed in better circumstances for its proliferation: a harbor town full of people, animals and cargo, many of which were on route to all ends of the known world. By then, news had, in fact, reached Moslems in the Near East that a devastating illness was killing the marmot trappers of central Asia and the dealers who sold their goods, but these reports were generally ignored in the West. It's well known traders carry not only exotic goods but also outlandish gossip.

When the containers with the marmot hides were opened in Kaffa, the rat fleas trapped within were released into an essentially defenseless population. Starting, no doubt, with the decimation of the local rats—but that's not likely to have made it into the historical record—there soon followed the infection and death of many other types of mammals none with significant resistance to this pathogen. Since people didn't rank high on that list because rat fleas prefer other animals like cats, dogs, and even cattle over humans, it took some time before the epidemic hit our species.

This initial delay was instrumental in the disease's ferocious progress. It ensured that Plague could establish itself on board the many ships leaving Kaffa every day. Here, historical documentation of the bubonic plague as a human disease finally begins to emerge. By late 1347, there is evidence of its presence in Constantinople, and soon thereafter Genoa in Italy and Messina in Sicily. The Byzantine Emperor Cantacuzenus watched it infect and consume his own son and, like the ancient Greek historian Thucydides, recorded a pathology, an account of its medical course.

Map: The Spread of the Black Death (click to see larger image)Out of fear of Plague, the Genoese—to their lasting discredit!—turned foreign ships away from their harbor, which not only accelerated the spread of the disease but did nothing to spare Genoa. As a rule, efforts to limit Plague in the Middle Ages served mainly to disperse it more widely, since Medieval quarantines involved sequestering the infected in a building. That only forced rats, fleas, humans and bacilli, the essential ingredients in Plague, into close proximity. As the Genoese of this day knew but never fully understood the significance, rats can swim off infected ships and, in doing so, carry fleas and bubonic plague with them.

Soon thereafter the Black Death appeared in Pisa (Italy) and Marseilles (on the southern coast of France). Nor did it spare the Moslem world, which first saw its ravages in Alexandria (Egypt), their great port city. From there, it moved east to Damascus and Beirut, and also west to Morocco and Spain. But the cleaner and generally more rat-free environs of Islamic communities, where medicine and health were far more advanced than in the West at that time, forestalled the spread of Plague eastward and it took relatively few victims there, at least compared to Western Europe.

By early 1348, the disease had begun to cut a swath west across France and descended on Bordeaux, a port in the Aquitaine region of southwestern France, famous for exporting wine. On a ship laden with claret, Plague reached England late that same year. In 1349, another ship, this one carrying English wool to Scandinavia, was spotted several days after it had departed its home port, floating aimlessly off the Norwegian coast. The locals rowed out to see it and found its crew dead but its cargo intact. They happily took the wool and, along with this treasure, infected fleas.

As if from some passage in the Old Testament giving witness to the eighth commandment, "Thou shalt not steal," Plague erupted with a vengeance across Scandinavia. From 1350 to 1352, it continued apace, ravaging Denmark, Germany, Poland and finally Russia. Thus, having made a five-year clockwise circuit of Europe, it ultimately passed back into the same remote Asian hinterland from which it had emerged originally, and disappeared. The Black Death itself was over, but the worst of it still lay ahead, the memories of its rampage and the crippling, nauseating fear it might return one day, as in fact it did sporatically over the next few centuries.

III. The Negative Consequences of the Black Death

The consequences of the Black Death on the culture of late Medieval Europe are immeasurable and, needless to say, mostly negative. By itself, the decrease in population forever changed the face of Western Civilization—the overall population of Europe would not surpass pre-1347 levels until after 1500—a century and a half to recover from what began as half a decade of human ruin puts the impact of this disease into its proper perspective. In terms of carnage alone, no war has even come close to that level of long-term devastation.

Burial of Victims (click to see larger image)Given the day and age, historians are hard pressed to produce reliable, even reasonable population figures. Nor does it help that prior to the Black Death many local governments had collapsed in the wake of the Great Famine of 1315-17 and the outbreak of the Hundred Years' War (1337-1453). Still, it's probably safe to say that something on the order of a quarter to a third of the population of Europe died during the Black Death, amounting to as many as twenty million people. Where the numbers of casualties can be calculated with any certainty—for instance, in urban centers like Paris—it's clear that between 1348 and 1444 the Black Death and recurrences of Plague cut the population by half, if not more.

The results of this contagion were, however, felt not in mortalities alone but in demographics and psychology, too. Grim experience quickly taught people in the day that Plague decimated cities more heavily than rural communities. The reason for this was that the bacillus depends on fleas carried by rats as its principal vector and the crush and filth of urban life aided greatly in the spread of bubonic plague, but that was not yet known. The result was that people fled the cities of Europe in large numbers. Even small villages were left depopulated, precipitating a trend toward de-urbanization far more catastrophic than that following Rome's disintegration a millennium before. And that, we should recall, had precipitated the Middle Ages.

This wave of de-urbanization and its concomitant catastrophes are well-evidenced in the art and literature of the day. Probably the most famous literary work of that time, The Decameron by Boccaccio, a collection of Medieval tales and folklore, is set in the Italian countryside where aristocrats, fleeing the Plague as it ravages Florence, are stranded without their usual entertainments. To pass the time, they tell each other stories, from which Boccaccio is said to have harvested a rich storehouse of traditional narrative. The Decameron later served as the foundation for many other Renaissance works, including several of Shakespeare's plays. Little wonder, then, so many of his dramas focus on death and the darker side of human life.

Durer's The Four Horsemen of the Apocalypse (click to see larger image)The visual arts of the day centered even more directly on the consequences of the Black Death. A macabre fascination with death and the process of dying fills painting and statuary from the fourteenth and fifteenth centuries. From these have sprung many of the images of death well-known today: the Grim Reaper, the "dance of death," and Albrecht Dürer's famous etching, "The Four Horsemen of the Apocalypse." Artists' emphasis on the democratic nature of death, which steals away both rich and poor, nobleman and peasant, pagan and priest, opened the door wide to a general questioning of the culture on which the Medieval synthesis had rested, such as the divine right of kings and the class constructs which tied serfs to the land. Offering little in the way of help—much less explanation or solace—these postulates began to crumble.

It also paved the way to extreme behavior. Staring down their mortality, many people gave into lewdness and revelry, while others turned to religion and extreme piety. In spite of the widespread devastation of both clergy and congregation, the Church ironically became richer than ever. More than one person in a desperate attempt to avert the Angel of Death surrendered all worldly possessions to the Church. When these prayerful gifts proved futile, the Church—and the papacy in Rome especially—ended up holding many a moneybag and deeds to land all over Europe. Thus, the failure of the Church to win divine mercy for its people turned out to be one of its greatest bull-markets ever, an irony not entirely lost on its laity.

Flagellants (click to see larger image)And so wherever the cry of "Plague!" was heard, despair manifested itself and not just in art and literature but also in bizarre social phenomena, one of which was flagellants. Professional self-torturers who went from town to town, the flagellants scourged themselves for a fee to bring God's favor upon a community hoping to avert the bubonic plague—according to Medieval logic, the Black Death was a punishment for sin, and its atonement must be paid in real, physical terms—flagellants served, then, as a means for people to buy that remission from sin at the price of migrant "whipping boys." The Church outlawed flagellants, though that did little to stifle them. Sickness and death of every sort, it seemed, followed fast on each other in a spiral of unending despair.

IV. The Positive Consequences of the Black Death

When the plague abates, the traffic gets worse. (Unknown satirist)

With all that, it may seem hard to believe but there were also positive consequences to the Black Death. Primarily, manpower was suddenly of much greater value than it had been before. For the first time in centuries, peasants weren't available in prodigious numbers and nobles had difficulty securing the workforce necessary to sow their fields and harvest their crops. Thus, the late Medieval peasant found himself quite unexpectedly and unprecedentedly in demand, a shift which shook European society to its core.

Kings and dukes now had to bargain with their laborers over working conditions, and the under-classes were able to demand better compensation for their services. Wages rose, in some places doubling over the course of just one year. At the same time, prices were falling because there were fewer people to buy goods. So, caught between rising production costs and falling revenue, middle-class lords tried to force a price-freeze and, when they couldn't, many gave up and sold their estates.

The resulting social upheaval accelerated trends in social evolution which had already been under way before the devastation. In particular, the Black Death terminated serfdom in Europe—serfs were virtual slaves, peasants who were "tied to the land" and obliged to farm certain areas for no other reason than that their ancestors had—the impact of Plague on society is clearly visible when one compares those places where it hit hard with those it didn't. In Russia, for instance, where the disease was never so destructive, serfdom continued as a social institution well into the nineteenth century. As such, Plague changed some things for the better.

The growth of workers' rights was, in turn, the stimulus for other social change in Europe, as laborers across the continent began to fight for their rights. For instance, in 1358 French workers, called collectively the Jacquerie, revolted in an effort to create better working conditions for peasants. Two decades later in 1378, Italian workers in Florence followed suit, and in 1381 the English did much the same in the Peasants' Revolt. If these upheavals resulted in little more than devastation and pillage, it proves only that workers and their leaders were not yet ready to take on the responsibilities of managing life in the mainstream, not that their pursuit of independence and self-governance was unjustified. There's no doubt that these attempts to assert common fairness and decency in the workplace foreshadow the evolution of modern labor unions. Thus, the Black Death precipitated some change for the good, at least among those of the working class who survived its onslaught.

Also, as the agriculturally oriented manorial system which had dominated life during the High Middle Ages slowly failed, industry rose, yet another benefit left in the wake of the Black Death. Once the major impact of the disease was no longer felt, the towns of Europe repopulated faster than smaller communities in the countryside. This new, urbanized Europe paved the way for a society and economy based on different principles, laying the groundwork for modern life, an era when cities, industry and trade have come to predominate over farming and living in the country.

And one other positive result of the bubonic plague was the development of medicine as a science in the West. Whereas in the late Middle Ages Islamic doctors had for centuries been advocating sensible measures like general cleanliness and the value of studying anatomy, Western healers prior to 1347 were still encumbered by the Medieval scorn of the body and ancient medical fallacies like the theory of humors. But when Plague wiped out nearly all the doctors in Europe, just as it had the clergy—physicians, like priests, attend to the dying and because of this were exposed at a higher rate to the more virulent pneumonic form of Plague—it precipitated a change in both personnel and precept. Ironically, then, modern Western medicine owes much to Yersinia pestis, one of its most horrifying failures.

V. Conclusion: The End of the Bubonic Plague?

DNA isn’t destiny---it’s history. . . . Somewhere in your genetic code is the tale of every plague, every predator, every parasite and every planetary upheaval your ancestors managed to survive. (Sharon Moalem, Survival of the Sickest)

A. The Post-Mortem of the Black Death

The Plague's assault on the West did not end with the Black Death. Long after 1352, buboes continued to swell intermittently across Europe—in 1369, 1374-5, 1379, 1390, 1407 and so on until as late as 1722—but the disease has never struck the modern world again with the force it did in 1347. Though particularly virulent outbreaks are recorded in 1665 in London and as late as 1896 in Bombay (Mumbai), the rate of infection and the percentage of the population killed always stopped before reaching the levels it had in the mid-fourteenth century and, more important, recurrences invariably turned out to be localized. This raises an important question: why hasn't Plague hit again as hard as it did when it launched the Black Death?

Historians and physicians alike have puzzled over this issue and, though many answers have been suggested, none has won general approbation. One is that the general hygiene of Europeans improved after the Middle Ages, but while people may, in fact, have started bathing more after the fourteenth century, rats and fleas which are central in spreading Plague did not adopt better standards of health. Fleas were certainly a persistent factor in human life until quite recently, so hygiene is not likely to be the reason Plague has never reappeared in as devastating a form as it was in the 1300's.

Rats (click to see larger image)Since rats are crucial in spreading Plague, other explanations have centered on them. Some scholars, for instance, have cited the relatively recent spread of brown rats across Europe—brown rats tend to live away from humans—as opposed to black rats which were more predominant earlier and usually live in or around human communities. This theory, however, does not hold up either, since the areas of Europe infested with brown rats do not coincide with those which evidence a reduction in the scope and impact of Plague.

Another explanation centering on rats is that the European species, both brown and black, developed a resistance to Plague. But that, too, seems unlikely since immunological resistance in a population, especially one with as high a birth and death rate as rats have, tends to dwindle over time. So, even if at some point their immunity to the disease increased, European rats should have become susceptible to Plague again fairly quickly.

A scientist named Colin McEvedy has proposed a new theory which seems to have some merit. According to McEvedy, the failure of Yersinia pestis to reappear in as virulent a form as it had in the fourteenth century depended on a change in the microbial world, not in humans or any mammalian species. Whether his thesis is right or wrong, it makes sense to look below the surface of visible life, since this disease operates principally on a microscopic, not macroscopic, level.

Respecting the durable dictum of pathology, that a "less virulent parasite will replace a more virulent parasite over time," McEvedy has suggested that after the Black Death European rats became less susceptible to Plague because Yersinia pseudotuberculosis, a bacillus closely related to Yersinia pestis but considerably less virulent, appeared in their environment. Exposure to this pathogen would have provided rat communities with some immunological resistance to Plague. That means, when Yersinia pestis re-appeared after the 1350's, the European rat population didn't die off as catastrophically as they had before, because some rats had acquired resistance to bubonic plague bacteria from having dealt with its milder, less often fatal counterpart, Yersinia pseudotuberculosis.

While humans were not exposed to this bacillus in any significant way and thus its appearance provided our species with no direct benefit, a growing immunity among rats to Yersinia pestis made the disease's journey from city to city more difficult. That is, too many rats across Europe had gained resistance to Plague for the pathogen to build up the momentum necessary to launch an all-out epidemic like the Black Death. And so while it continued to flare up on occasion, bubonic plague failed to sweep the continent ever again the way it did in the mid-fourteenth century.

B. The Future of Plague

With that, it would seem we have finally reached the end of the history of the Black Death, but in fact we have not. For one, though controlled by antibiotics and much suppressed, bubonic plague is still a factor in human life. Even today, it remains endemic in Uganda, the western Arabic peninsula, Kurdistan, northern India and the Gobi desert, and lately there have been ever increasing numbers of cases documented in the United States, particularly among hunters of rockchucks in the American West. Moreover, the possibility always exists that through some mutation Yersinia pestis could once again rampage through rats and other mammals and, if it gains the ability to resist antibiotics, devastate the human population as well.

At the moment, however, that seems unlikely, and the work of modern medical researchers centers more on the plagues which threaten and ravage the world today: AIDS, Ebola, Dengue fever, avian flu and the like. These, for the most part, stem from viruses, not bacteria, and draw attention toward the effort to find cures for viral infections. Recent research, however, has shown that the barrier between the world of the virus and the bacillus is not as impermeable as it might seem. Statistical analysis of AIDS mortalities has turned up an intriguing connection between the diseases plaguing us today and the one our Eurasian predecessors endured. To wit, data suggest that people whose ancestors come from those areas of Europe which suffered most heavily during the Black Death coincide with populations today which exhibit lower rates of mortality from AIDS.

If this thesis is correct, it means that the exposure of their ancestors to Plague enhances the possibility that certain peoples will in general be able to resist AIDS more effectively. Thus, the past indeed has great bearing on the present—and the future!—and as the report about this theory says, "it will add to a growing recognition among scientists of the importance of epidemics in shaping human evolution." That's something all competent historians, no matter their ancestry, could have told you long ago.

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