Thirdly: long-standing local customs among natives of Yunnan and Manchuria alike seem to have inhibited the transfer of bubonic infection to human beings quite effectually, despite the endemic presence of Pasteurella pestis in rodent burrows of those regions. Only when newcomers failed to observe local “superstitions” did plague become a human problem. Moreover, in both regions the irruption of epidemiologically uninformed strangers was associated with military-political upheavals of the sort that have often provoked disease disasters in times past.
In view of the apparent effectiveness of traditional customary safeguards against plague in Yunnan and Manchuria, one may recognize that the medical prophylaxis developed so successfully between 1894 and 1924 was a quite normal, though unusually speedy and effective, human response to epidemiological emergency. Instead of allowing myth and custom, through a process of trial and error, to define an acceptable style of human behavior that would keep disease within tolerable limits, as men had always done before, scientific medicine arrived at new rules of conduct and employed a worldwide political frame—international quarantine regulations—to compel general acquiescence in newly prescribed behaviors. In such a perspective the brilliant triumphs of twentieth-century medical science and public health administration do not seem quite so novel as they otherwise might; though the fact remains that the medical discoveries of this century with respect to bubonic plague far surpass the efficiency of earlier behavior patterns for containing the ravages of the disease. Doctors and public health officers, in fact, probably forestalled epidemics that might have checked or even reversed the massive worldwide growth of human population that distinguishes our age from all that have gone before.8
With this modern and carefully observed example in mind, let us return to the thirteenth century, and consider what probably happened to the distribution of Pasteurella pestis in Eurasia as a consequence of the new patterns of human movement that the Mongols inaugurated. We must assume that prior to the Mongol conquests the plague was endemic in one or more natural foci among communities of burrowing rodents. In such regions human populations had presumably arrived at a customary pattern of behavior that minimized chances of infection. As we saw in the preceding chapter, one such natural focus was probably located in the borderland between India, China, and Burma in the Himalayan foothills; another probably existed in central Africa in the region of the Great Lakes. The Eurasian steppelands between Manchuria and the Ukraine, however, were almost certainly not yet a focus of plague.
This becomes evident if one compares the history of plague after its first devastating appearance in Europe during the age of Justinian with what happened after 1346 when the Black Death arrived. In the first instance, plague eventually disappeared entirely from Christian Europe. The last mention of the disease in Christian sources dates from 767.9 Arab writers, likewise, make no mention of plague for at least 150 years before the 1340s.10 One must therefore assume that after a series of precarious transfers from city to city within the Mediterranean region, the chain of infection among rats, fleas, and humans broke off because Pasteurella pestis had failed to find a stable, ongoing ecological niche within which it could abide lastingly.
By contrast, ever since 1346 plague has remained chronic in Europe and the Middle East, right down to the present.11 Even when northwestern Europe ceased to suffer from the plague in the seventeenth century, eastern Europe continued to be exposed. Moreover, in the eighteenth century, when consular reports allow a quite exact reconstruction of the plague history of the busy port of Smyrna in Asia Minor, it is clear that the disease came via caravan from the interior (i.e., from the Anatolian plateau or steppelands beyond) and spread from Smyrna by sea to other ports. The continued seriousness of the infection can be surmised from the fact that between 1713 and 1792 only twenty years were entirely plague-free in Smyrna, and in the nine periods of epidemic, death tolls ranged up to 35 per cent of the entire population of the city.12
The contrast between Europe’s recurrent experience of the plague after 1346 and the apparent absence of the disease from European soil for more than five and a half centuries before 1346 indicates that something drastic had happened to enhance Europe’s exposure to the infection. In view of what is known of how the plague bacillus seized the opportunities that nineteenth-century steamships offered for expanding its radius of action, it seems probable that in the fourteenth century Pasteurella pestis somehow did likewise and penetrated the rodent populations of the Eurasian steppe for the first time, thereby inaugurating the endemic infections medical men in the 1920s discovered to exist in Manchurian and Ukrainian rodent burrows.
Nor is it hard to find circumstances that might have permitted the transfer of Pasteurella pestis from its earlier endemic focus in the Himalayan foothills to the broad northern grasslands of Eurasia. For in the second half of the thirteenth century, Mongol horsemen penetrated Yunnan and Burma (beginning 1252–53) and thereby entered the regions where wild rodents today play host to the plague bacillus on a chronic basis, and where similar infection had probably existed for many centuries before the Mongols arrived. As in 1855, when unusual military movement allowed Pasteurella pestis to cross the Salween and begin its nineteenth-century peregrination around the world, so also in the thirteenth century, it is entirely probable that the Mongol invaders disregarded whatever local rules and customs had arisen to insulate human populations from bubonic infection. Like Chinese marmot trappers of the twentieth century, they therefore presumably infected themselves and thus inadvertently allowed the disease to break through former geographic limits.
The superior speed mounted horsemen commanded meant that the infection was able to extend its range of action in the thirteenth century just as it later did in the nineteenth and twentieth centuries. Infected rats and fleas could, at least occasionally, hitch a ride in a saddlebag stuffed with grain or some other form of booty, and the rapidity with which Mongol military detachments habitually moved meant that rivers and similar barriers to the slow diffusion of the infection could now be crossed as easily as oceans later were to be. It does not require a very great leap of the imagination, therefore, to believe that some time after 1252, when the Mongols first invaded Yunnan-Burma, they inadvertently transferred the plague bacillus to the rodent population of their own steppe homeland and thereby inaugurated the chronic pattern of infection which medical researchers discovered in Manchuria in our own time.
Exactly when and how this geographic transfer occurred cannot, of course, be exactly discerned, any more than the paths by which bubonic infection reached the wild rodents of California or Argentina can be precisely described. On the analogy of events in the nineteenth and twentieth centuries we might suppose that infection of rodent underground cities in the steppelands began soon after the time when Mongol conquerors first erected a bridge of moving horsemen between Yunnan-Burma and Mongolia in the mid-thirteenth century. To be sure, infection in Mongolia was not equivalent to infection of the entire steppe. That would take time. We might, thus, imagine that for nearly a hundred years Pasteurella pestis moved across the Eurasian steppelands from one rodent community to another in the same fashion as occurred in North America after 1900.
One hypothesis, therefore, is that soon after 1253, when Mongol armies returned from their raid into Yunnan and Burma, Pasteurella pestis invaded the wild rodent communities of Mongolia and became endemic there. In succeeding years the infection would then spread westward along the steppe, perhaps sporadically assisted by human movement, as infected rats, fleas, and men inadvertently transferred the bacillus to new rodent communities. Then, shortly before 1346, the pool of endemic rodent infection perhaps began to reach its natural limits.13
On the whole, however, this reconstruction of events seems improbable. The trouble is that Chinese records do not show anything unusual before 1331, when an epidemic in the province of Hopei is said to have killed nine tenths of the population. Not until 1353–54 do available records indicate a more widespread disas
ter. In those years epidemic disease raged in eight different and widely scattered parts of China, and chroniclers reported that up to “two thirds of the population” died.14 Even allowing for interruptions of record keeping resulting from local disorder and the breakdown of administrative routines during the prolonged Mongol conquest of China (1213–79), it seems hard to believe that any really massive disease die-off would have escaped the antiquarian compilers, whose lists of disasters provide the basis for the only available information about Chinese epidemics.
Perhaps a careful and epidemiologically informed study of all surviving Chinese texts—and they are extraordinarily voluminous—will cast more light on the question some day. But until such a search has been carried out, one must, I think, assume that the plague that burst so lethally upon Europe in 1346 manifested itself in China no earlier than 1331. And if this is so, one cannot easily believe that Pasteurella pestis had found a new lodgment in steppe burrows as early as the 1250s. If that had happened, China’s encounter with plague would have begun long before 1331, and the vast cities and dazzling splendor of Kublai Khan’s court (reigned 1257–94), as reported to us by Marco Polo, could scarcely have flourished as they did.
By contrast, after 1331, and more particularly after 1353, China entered upon a disastrous period of its history. Plague coincided with civil war as a native Chinese reaction against the Mongol domination gathered headway, climaxing in the overthrow of the alien rulers and the establishment of a new Ming Dynasty in 1368. The combination of war and pestilence wreaked havoc on China’s population. The best estimates show a decrease from 123 million about 1200 (before the Mongol invasions began) to a mere 65 million in 1393, a generation after the final expulsion of the Mongols from China.15 Even Mongol ferocity cannot account for such a drastic decrease. Disease assuredly played a big part in cutting Chinese numbers in half; and bubonic plague, recurring after its initial ravages at relatively frequent intervals, just as in Europe, is by all odds the most likely candidate for such a role.
This interpretation of Chinese records fits well with what the best-informed contemporary observers in Europe and the Near East were able to find out about the origins of the plague. A Moslem writer, Ibn al-Wardi, who lived through the initial onslaught of the plague in Aleppo, remarked that the disease originated in the “Land of Darkness” and spread through northern Asia before invading the civilized world, starting with China, and proceeding thence to India and the realm of Islam.16 Aleppo, being a caravan city itself, and a key point in the complicated network of trade that ran across the grasslands of Asia in the fourteenth century, was a particularly good place from which to get an accurate account of the spread of the plague. A Christian inquiry into the pre-history of the Black Death arrived at the conclusion that the plague had first appeared in China (the second station in al-Wardi’s account of the peregrinations of the disease) and had spread thence across Asia to the Crimea.17
What seems most likely, therefore, is that Pasteurella pestis invaded China in 1331, either spreading from the old natural focus in Yunnan-Burma, or perhaps welling up from a newly established focus of infection among the burrowing rodents of the Manchurian-Mongolian steppe. The infection must then have traveled the caravan routes of Asia during the next fifteen years before reaching the Crimea in 1346; whereupon the bacillus took ship and proceeded to penetrate almost all of Europe and the Near East along routes radiating inland from seaports.
Assuredly, the far-flung network of caravanserais extending throughout central Asia and eastern Europe offered a ready-made pathway for the propagation of Pasteurella pestis across thinly inhabited regions. Each regular resting place for caravans must have supported a complement of rats and fleas, attracted there by the relatively massive amount of foodstuff necessary to keep scores or even hundreds of traveling men and beasts going. Such populations of rats and fleas stood ready, like similar concentrations of rats at gristmills in the interior parts of western Europe, to receive and propagate Pasteurella pestis whenever it might appear, whether introduced initially by rat, flea, or human carrier. Then, when the humanly lethal consequences of the local propagation of the infection became apparent, one can be certain that everyone able to flee would do so, thus transferring the bacillus to some new, similar locus for still further propagation.18
On this assumption, the spread of Pasteurella pestis to underground rodent “cities” of the Eurasian steppe, where the infection was to find lasting, stable hospitality, occurred in a much shorter period of time than would have been needed if the bacillus had spread as it did in the United States from one rodent community to another without significant human involvement. An isolated bit of evidence supports the hypothesis of a rapid diffusion across Eurasia, for in 1338–39 an epidemic broke out in a Nestorian community of traders in central Asia near Issyk Kul. A Russian archaeologist dug up their bones and, relying on a statistical analysis of burials and some ancient texts, concluded that bubonic plague had been responsible for the deaths.19
What probably happened between 1331 and 1346 therefore, was that as plague spread from caravanserai to caravanserai across Asia and eastern Europe, and moved thence into adjacent human cities wherever they existed, a parallel movement into underground rodent “cities” of the grasslands also occurred. In human-rat-flea communities above ground, Pasteurella pestis remained an unwelcome and lethal visitor, unable to establish permanent lodgment because of the immunity reactions and heavy die-off it provoked among its hosts. In the rodent burrows of the steppe, however, the bacillus found a permanent home, just as it was later to do among the burrowing rodent communities of North America, South Africa, and South America in our own time.20
Yet epidemiological upheavals on the Eurasian steppe, whatever they may have been, were not the only factors in Europe’s disaster. Before the Black Death could strike as it did, two more conditions had to be fulfilled. First of all, populations of black rats of the kind whose fleas were liable to carry bubonic plague to humans had to spread throughout the European continent. Secondly, a network of shipping had to connect the Mediterranean with northern Europe, so as to be able to carry infected rats and fleas to all the ports of the Continent. Very likely the spread of black rats into northern Europe was itself a result of the intensification of shipping contacts between the Mediterranean and northern ports. These date, on a regular basis, from 1291, when a Genoese admiral opened the Strait of Gibraltar to Christian shipping for the first time by defeating Moroccan forces that had hitherto prevented free passage.21 Improvements in ship design occurring in the thirteenth century made year-round sailing normal for the first time, and rendered the stormy Adantic safe enough for European navigators to traverse even in winter months. Among other things, ships constantly afloat offered securer and more far-ranging vehicles for rats. Consequently rat populations could and did spread far beyond the Mediterranean limits that seem to have prevailed in Justinian’s time.
Finally, many parts of northwestern Europe had achieved a kind of saturation with humankind by the fourteenth century. The great frontier boom that began about 900 led to a replication of manors and fields across the face of the land until, at least in the most densely inhabited regions, scant forest remained. Since woodlands were vital for fuel and as a source of building materials, mounting shortages created severe problems for human occupancy. In Tuscany, collision between an expanding peasant population and the agricultural resources of the land seems to have occurred even earlier, so that a full century before the Black Death struck, depopulation had begun.22 On top of this, the climate worsened in the fourteenth century, so that crop failures and partial failures became commoner, especially in northerly lands, as the length and severity of winters increased.23
All these circumstances converged at the middle of the fourteenth century to lay the basis for the shattering experience of the Black Death. The disease broke out in 1346 among the armies of a Mongol prince who laid siege to the trading city of Caffa in the Crimea. This compelled his withdrawal
, but not before the infection had entered Caffa itself, whence it spread by ship throughout the Mediterranean and ere long to northern and western Europe as well. (See map.)
The initial shock, 1346–50, was severe. Die-offs varied widely. Some small communities experienced total extinction; others, e.g., Milan, seem to have escaped entirely. The lethal effect of the plague may have been enhanced by the fact that it was propagated not solely by flea bites, but also person to person, as a result of inhaling droplets carrying bacilli that had been put into circulation by coughing or sneezing on the part of an infected individual.24 Infections of the lungs contracted in this fashion were 100 per cent lethal in Manchuria in 1921, and since this is the only time that modern medical men have been able to observe plague communicated in this manner, it is tempting to assume a similar mortality for pneumonic plague in fourteenth-century Europe.
Whether or not pneumonic plague affected Europeans in the fourteenth century, die-off remained very high. In recent times, mortality rates for sufferers from bubonic infection transmitted by flea bite has varied between 30 and 90 per cent. Before antibiotics reduced the disease to triviality in 1943, it is sobering to realize that in spite of all that modern hospital care could accomplish, the average mortality remained between 60 and 70 per cent of those affected.25
Despite such virulence, communications patterns of medieval Europe were not so closely knit that everyone was exposed, even though an errant ship and infected rat population could and did bring the plague to remote Greenland and similarly distant outliers of the European heartlands.26 Overall, the best estimate of plague-provoked mortality, 1346–50, in Europe as a whole is that about one third of the total population died. This is based on a projection upon the whole Continent of probable mortality rates in the British Isles, where the industry of two generations of scholars has narrowed the range of uncertainty to a decrease in population during the plague’s initial onset of something between 20 and 45 per cent.27 Transferring British statistics to the Continent as a whole at best defines an approximate magnitude for guess-estimation. In northern Italy and French Mediterranean coastlands, population losses were probably higher; in Bohemia and Poland much less; and for Russia and the Balkans no estimates have even been attempted.28, 29
Plagues and Peoples Page 18
