by John Kelly
Among the species that do develop at least a partial immunity to
Y. pestis is its host population, rodents. Indeed, the plague bacillus’s exquisite attunement to the rodent community is one of the great marvels of nature. Most of the time
Y. pestis and the rodent kingdom live in a state of unhappy but workable coexistence. The scientific term for the modus vivendi is enzootic. Animals continue to get sick and die, but usually there are enough partially resistant rodents in any given community to keep the smoldering embers of infection in check. There are a number of theories about why, from time to time, this biological firewall suddenly collapses and the colony is consumed by the flames of an epizootic—a full-scale outbreak of plague. These include genetic change in the plague bacillus, which makes it more virulent, and demographic changes in the wild rodent community, which make its members more vulnerable to plague. A third theory, not incompatible with the first two, is that epizootics are activated by surge years—sudden, dramatic spurts in the rodent population. No one is sure what causes surges, but a number of scientists believe that they may be related to sun spots, which follow roughly the same cyclical pattern as surge years in many (though not all) rodent species, approximately ten to twelve years.
The connection is not as odd as it sounds. Sun spot cycles—which influence rainfall, tropical cyclones, and tree growth—may affect the wild rodent food supply. Climate changes may make vegetation more abundant, encouraging a burst of overbreeding, perhaps by affecting rodent fertility. Certain species of hare are known to experience cyclical bursts of fecundity. Clearer is what happens during a surge year; rodent populations breed themselves into a classic Malthusian dilemma: too many animals, too little food. And, as Malthus noted, when that happens nature usually prunes the population back to a sustainable level by way of a major demographic catastrophe, such as a famine or infectious disease. In the case of rodents, one component of the pruning mechanism may be an alteration in the community’s genetic composition. As rodent numbers spike during the surge, the pool of older, partially immune animals—the community firewall—is diluted by a rapidly expanding group of younger animals who have not yet acquired resistance to
Y. pestis. This pool of unprotected young may constitute the biological equivalent of an oil slick; throw a match on it and it bursts into flame.
Aside from providing an insight into nature’s secret harmonies, the dynamics of rodent populations would not matter much to humans but for the fact that during surge years towns, villages, and campsites are more likely to be invaded by hungry rodents. In one thirty-four-year period on the eastern steppe, four of five plague outbreaks occurred in tarabagan surge years, and the victims were local hunters, men schooled in the dangers of trapping sick animals. If experienced steppe veterans proved vulnerable to
Y. pestis, it does not take a large leap to imagine what would happen to a group of unwary outsiders unlucky enough to brush up against a tarabagan community in Mongolia, Manchuria, or Siberia in the midst of a surge year, particularly if the tarabagans’ food supply was under threat not only from the demographic pressures of the surge but also from long-term ecological changes.
Indeed, it is not even necessary to imagine what might happen; a historical precedent is available. Between 1907 and 1910, the world price of tarabagan skins quadrupled from 0.3 rubles to 1.2 rubles, producing a corresponding increase in the number of tarabagan hunters. Many of the newcomers were unskilled Chinese, looking to turn a fast ruble and ignorant of steppe lore about hunting staggering tarabagan. In April 1910 pneumonic plague broke out among a colony of hunters in Manchuria; within a year sixty thousand people were dead.
In the case of the Black Death, the first group of outsiders to be infected may have been Mongol herdsmen looking for new pastureland. During the fourteenth century, the wind patterns of Eurasia changed, altering the climate of the continent; Europe became wetter, while Asia became drier. The arrival of desertlike weather in the East may have forced Tartar herdsmen out of their traditional pasturelands and into the “tarabagan gardens” of the northern steppe, a region whose dangers they were as ignorant of as were the Chinese hunters in early-twentieth-century Manchuria. From the herdsmen, the plague would have spread to other outsiders: Arab, Persian, Italian, or Central Asian merchants; Tartar horsemen and soldiers; Chinese or Ukrainian laborers—or some combination of all or most of the above. Also easy to imagine is how political and economic changes like the rise of the Mongol Empire and the development of a nascent global economy would have allowed
Y. pestis to overcome the vast distances, thin populations, and other firewalls that had kept it trapped in the remoteness of inner Asia for centuries. After unifying the fractious steppe under a
pax Mongolica, the Tartars threw several overlapping communication networks across the vast, open prairies of Asia and Russia, including the Yam, a pony express service, new trade routes, and a network of caravansaries.
William McNeill, author of
Plagues and Peoples, thinks the caravan rest stops may have played a key role in the early spread of plague. “Assuredly the far-flung network of caravanserais extending throughout Central Asia . . . made a ready made pathway for the propagation of [plague] across thinly inhabited regions. Each resting spot for caravans must have supported a complement of fleas and rats attracted there by the relative massive amount of foodstuffs necessary to keep scores even hundreds of travelling men and beasts going.”
If the Black Death originated in or near the Gobi desert,
Y. pestis would have visited a half dozen such rest stops before climbing a mile in the sky to Lake Issyk Kul, the hot lake where it first burst into history. Warmed by thermal springs that can produce water temperatures of 85 to 95 degrees Fahrenheit, Issyk Kul sits five thousand feet above sea level in a bowl of snow-capped mountain and thick forest. Today the lake region is full of ghosts—Soviet, czarist, even a few Tartar, including the ripply outlines of a submerged village a few hundred feet from the shoreline. But in the mid-fourteenth century, Issyk Kul was a bustling trading center astride the northern steppe route. From the region, eastbound travelers could pick up the fast road into China; westbound travelers, the road home to Caffa, Tabriz, or Baghdad. Medieval Issyk Kul also had a substantial local population. Many of its inhabitants were Nestorians, a Christian sect of Syrian origin that spread across Asia in the early Middle Ages. On arriving in China, evangelical pioneers like John of Monte Corvino were astonished to see church spires rising above cities like Hangchow. “We have found many Christians scattered all over the east and many fine churches, lofty, ancient and of good architecture,” declared one Western visitor.
The Nestorians were both an accomplished and a flowery people. The inscriptions on the headstones found in two local Issyk Kul cemeteries speak in the florid language of the funeral oration. One informs the passerby: “This is the grave of Shliha, the celebrated commentator and teacher, who illuminated all the monasteries with light. . . . His voice rang as high as a trumpet.” Another: “This is the grave of Pesoha, the renowned evangelist and preacher who enlightened all. . . . Extolled for wisdom and may our Lord unite his spirit with the saints.”
In comparison, the inscription on a third Nestorian headstone, that of a husband and wife, Kutluk and Magnu-Kelka, has an almost ominous starkness. No accomplishments are mentioned, no holiness praised. The headstone tells us only enough to suggest the following scenario: one morning in 1339, perhaps a fragrant early-summer morning when the air temperature almost matched the water temperature on the lake, Kutluk awoke with the early symptoms of plague. On that first day he felt lightheaded and nauseous, symptoms so unobtrusive Magnu-Kelka did not even realize her husband was ill until dinner, when Kutluk suddenly vomited into his meal. On the second day of his illness, Kutluk awoke with a terrible pain in his groin; overnight, a hard, apple-sized lump had formed between his navel and his penis. That afternoon, when Magnu-Kelka probed the tumor with a finger, the pain was so terrible, K
utluk rolled over on his side and vomited again.
Toward evening, Kutluk developed a new symptom; he began to cough up thick knots of bloody mucus. The coughing continued for several hours. As night gathered around the lake, a sweaty, feverish Kutluk fell into a delirium; he imagined he saw people hanging by their tongues from trees of fire, burning in furnaces, smothering in foul-smelling smoke, being swallowed by monstrous fish, gnawed by demons, and bitten by serpents. The next morning, while Kutluk was reliving the terrible dream, the cough returned—this time even more fiercely. By early afternoon, Kutluk’s lips and chin had become caked with blood, and the inside of his chest felt as if it had been seared by a hot iron. That night, while Magnu-Kelka was sponging Kutluk, the tumor on his groin gurgled. For a moment Magnu-Kelka wondered if the swelling were alive; quickly, she made the sign of the cross. On the fourth day of his illness, Kutluk stained his straw bed with a bloody anal leakage, but Magnu-Kelka failed to notice. After vomiting twice in the morning, she slept until dark. When she awoke again, it was to the sound of crickets chirping in the evening darkness; she listened for a moment, then vomited on herself. On the fifth day of his illness, Kutluk was near death. All day Magnu-Kelka lay on a straw mat on the other side of the cottage, listening to her husband’s hacking cough and breathing in the fetid air. Toward evening Kutluk made a strange rattling sound in his throat and the cottage fell silent. As Magnu-Kelka gazed at her husband’s still body, she felt an odd sensation—like the fluttering of butterfly wings against the inside of her chest. A moment later, she began to cough.
Kutluk and Magnu-Kelka were almost certainly not the first victims of the Black Death, but their remote little lakeside cottage is where the most terrible natural disaster in history begins to enter the human record.
Two other notable names in the history of plague are Justinian, the sixth-century Byzantine emperor, and Alexandre Yersin, a dreamy young Swiss scientist who became the Yersin in
Yersinia pestis during the Third Pandemic of a century ago. The human equivalent of an epizootic, a pandemic is a catastrophic outbreak of infectious disease; including the Black Death, three times in recorded human history plague has risen to the level of pandemic. The first time, the Plague of Justinian, is where the story of man and
Y. pestis begins, and the last time, the Third Pandemic, is where the mysteries of the plague bacillus were finally unraveled. The reemergence of large-scale pestilence in late-nineteenth-century China horrified the self-confident Victorians, occasioning an early example of what today is called Big Science. In the 1890s, as
Y. pestis swept through China and India, researchers from dozens of countries focused their energies on a single, urgent question: “What causes plague?”
In the end, the worldwide race narrowed down to one city, Hong Kong, and two young men, each a surrogate for the two great microbiologists of the Victorian age, the Frenchman Louis Pasteur and the German Robert Koch. Koch’s surrogate, a former student named Shibasaburo Kitasato, was a heavyset, ambitious young man who wore a starched wing-tip collar even in the sultry Hong Kong heat and enjoyed the seemingly unbeatable advantages of modern equipment, a large staff, and devious mind. Pasteur’s surrogate was the moony Yersin, a Somerset Maugham-ish figure, who gave up a life of privilege in the West to search for Higher Truth in the East. In a film about the race to identify
Y. pestis, Leslie Howard would have played Yersin.
Despite having fewer resources and lacking the inspired duplicity of Kitasato (“The Japanese . . . have bribed the staff of the hospital so that they will not provide me with [any bodies for] autopsy!” Yersin complained to his mother), the young Swiss investigator became, in 1894, the first person to accurately describe the plague pathogen. “The pulp of the buboes always contains short, stubby bacilli,” he noted in one of the most important papers ever written about human disease. A few years later a Frenchman named Paul-Louis Simond identified the rat and rat flea,
X. cheopis, as plague vectors. In 1901 Kitasato’s mentor, Robert Koch, summarized the new research this way. Plague, said Koch, is “a disease of rats in which men participate.” A few decades later the first effective antiplague medications began to become available.
If the Third Pandemic is where the story of man and plague ends—at least for now—then the sixth-century Plague of Justinian is where it begins. There are several references to what sounds like plague in the Bible, but
Y. pestis did not officially enter human history until a.d. 542, when it strode off a ship in the Egyptian port of Pelusium. From a modern perspective, the most striking thing about the Plague of Justinian is how closely it resembles the Black Death. There is, first of all, the crucial role of commerce in disseminating disease. Until a trade route from Egypt made it more accessible, Ethiopia, the probable home of the First Pandemic, was as remote from the major population centers of late Antiquity as the other plague foci of the premodern world: the Eurasian steppe (including Siberia and the Gobi desert), Yunnan in China, and perhaps Kurdistan in Iran. Like the Black Death, the Plague of Justinian also occurred during a period of extreme ecological change. A recent study of two thousand years of tree ring data reveals that four years in the last two millennia were periods of extraordinarily severe weather, and two of those four years were situated in or around a plague pandemic. One was 1325, roughly the time
Y. pestis may have been at work on the rodent population of the Gobi or some other region of inner Asia; the other was 540, two years before
Y. pestis arrived in Pelusium, and roughly around the time that Chinese scribes were describing yellow dust falling like snow and Europeans were complaining about the bitter cold that ushered in the Dark Ages. “The winters [are] grievous and more severe than usual,” wrote the sixth-century monk Gregory of Tours. “The streams are held in chains of frost and furnish . . . a path for people like dry ground. Birds too [are] affected by the cold and hunger and [are] caught in the hand without any snare when the snow [is] deep.”
In the Plague of Justinian, one also hears for the first time a sound that becomes overpowering during the medieval pestilence: the sound of human beings drowning in death. “At the outset of this great misfortune,” wrote a lawyer named Evagrius, “I lost many of my children, my wife and other relatives and numerous estate dwellers and servants. . . . As I write this in the 58th year of my life . . . I [have recently] lost another daughter and the son she has produced quite apart from other losses.”
No one knows how many people died in the Plague of Justinian, but in Constantinople, where the daily mortality rate is said to have reached ten thousand at the peak of the epidemic, people put on name badges so that they could be identified if they fell on the streets. The mortality was also very severe in the Middle East. “In every field from Syria to Thrace the harvest lacked a harvester,” wrote John of Ephesus, who went to bed each night expecting to die and awoke each morning surprised to find himself still alive. Untold thousands also perished in Italy and in France, where Gregory of Tours reported that “soon no coffins or bearers were left.”
The Plague of Justinian marked an important turning point in Europe’s relationship with infectious disease. The centuries preceding the First Pandemic were a period of chronic, devastating epidemics. In the second and third centuries, smallpox and measles outbreaks may have killed a quarter to a third of the population in parts of the Roman Empire. The centuries after Justinian were, if not disease-free, then close to it. During the early Middle Ages, all forms of infectious illness became uncommon and plague (as far as is known), nonexistent. For this disease-free interim, the collapse of civilization deserves some credit. A disease is more than just a pathogen plus a transportation system. To ignite an epidemic, a friendly environment is also necessary, and after Rome fell, the environment in Europe, particularly northwest Europe, became increasingly unfriendly to epidemic disease. In the early Middle Ages, the population plunged precipitously, shrinking the pool of potential host-victims. In the second and third c
enturies, Roman Europe had 50 to 70 million people; by 700, Europe had 25 to 26 million. The disappearance of urban life removed two other necessities: concentrations of people, and filthy, rodent-infested streets. At its height, the city of Rome had a population variously estimated at half a million to ten million; by 800, no city in Europe contained more than twenty thousand residents. “In the middle of the debris of great cities,” wrote one Dark Ages scribe, “only scattered groups of wretched peoples survive.”
The resurgent forest provided a further barrier against infectious illness. By a.d. 800, dense woodland had reclaimed 80 percent of a depopulated Europe’s surface, severely restricting trade and travel and providing a firewall, which helped to keep local epidemics local. The international situation added a final layer of insulation. By the ninth century, the principal east-west trade routes were all in unfriendly Muslim hands.
Around the year 1000, this process began to reverse itself and Europe started to re-create the environmental conditions associated with demographic collapse in premodern societies. And indeed, four hundred years later the West would suffer a second great demographic catastrophe, but that gets us ahead of our story, which begins where stories of pandemics often do: in a burst of human progress.
Sometime between 750 and 800, Europe entered the Little Optimum,* a period of global warming. Across the continent, temperatures increased by an average of more than 1 degree Celsius, but, rather than producing catastrophe, as many current theorists of global warming predict, the warm weather produced abundance.* England and Poland became wine-growing countries, and even the inhabitants of Greenland began experimenting with vineyards. More important, the warm weather turned marginal farmland into decent farmland, and decent farmland into good farmland. In the final centuries of Roman rule, crop yields had fallen to two and three to one—a yield represents the amount of seed harvested to the amount planted: a return so meager, the Roman agricultural writer Columella feared that the land had grown old. In the eleventh and twelfth centuries, as winters became milder and summers warmer and drier, European farms began to produce yields of five and six to one, unprecedented by medieval standards.
