Plagues and Peoples

by William H. McNeill

  To be sure, on the west a narrow Aegean gateway offered a prospect of expansion onto fresh and sufficiently fertile ground for supporting the imperial style of macroparasitism. But when Xerxes’ armies tried to make this possibility real, 480–479 B.C., they met defeat, as much from difficulties of supply as from the valor of the leagued Greek cities that resisted the Persian invasion. A similar gateway existed far to the southeast in the Indian Doab, a fertile region between the up waters of the Indus and Ganges. The Persians made no recorded attempt to force this gate, however, and when Alexander of Macedón did try in 326 B.C., his troops mutinied and refused to follow. As a matter of fact, a disease gradient that assured severe losses to any army invading from beyond the Himalayas was probably more effective in guarding this gateway than any merely human obstacle.

  Can we also say that microparasitism achieved a kind of natural limit within the expanded circuit of Middle Eastern civilized society by about 500 B.C.? Perhaps the forms of parasitism appropriate to irrigation farming and dependent on the specialized exposures to infection and infestation resulting from frequent wading in irrigation water had attained a fairly stable balance by 500 B.C. Irrigation farming was at least 3,000 years old by then, and communication between its major centers in Egypt, Mesopotamia and the Indus Valley was sufficient to have permitted a thorough homogenization of parasitic organisms across the 2,000–3,000 years during which these valleys had maintained contact with each other. Absence of evidence in written sources of any notable change in worm and related forms of infestation can scarcely be taken as confirmatory, since those who wrote paid practically no attention to the life conditions of the peasantry, and medical texts are completely opaque when it comes to translating ancient terms into modern disease classifications.

  Written evidence does, however, clearly attest the appearance of epidemic diseases in the ancient Middle East. Among the disasters mentioned in the Babylonian Epic of Gilgamesh as preferable to the Flood was visitation from the god of pestilence, and an Egyptian text of about the same age (ca. 2000 B.C.) compares fear of Pharaoh with fear of the god of disease in a year of pestilence.1 In China, too, the most ancient decipherable writings, dating back to the thirteenth century B.C., show familiarity with infectious epidemic disease. “Will this year have pestilence and will it be deaths?” asked an ancient ruler of Anyang.2 His expert diviners thereupon recorded the question in a form that modern scholars can read on the sheep’s shoulderblade used in ritually seeking an answer from the spirits.

  Biblical texts are of substantially later date but may preserve oral traditions going back to about the same time. There may therefore well have been a historical basis for the plagues of Egypt described in the Book of Exodus. It is there stated that among the plagues Moses brought down upon Egypt were “sores that break into pustules on man and beast.”3 Furthermore, a lethal visitation upon Egypt’s first-born in a single night left “not a house where there was not someone dead.”4 One may also cite an epidemic visited on the Philistines as punishment for their seizure of the Ark; the pestilence that punished David’s sin of numbering the people, and killed, if the text of the Bible is to be believed, 70,000 out of 1, 300,000 able-bodied men in Israel and Judah; and the fatal visitation that “slew in the camp of the Assyrians one hundred and eighty-five thousand” overnight, and caused the Assyrian king, Sennacherib, to withdraw from Judah without capturing Jerusalem.5, 6, 7

  Such passages make it certain that the writers of the Old Testament, when they put the text into its present form between 1000 and 500 B.C., were quite familiar with the possibility of a sudden outbreak of death-dealing disease, and interpreted such epidemics as acts of God. Modern translators regularly used the term plague for such events, since the principal disease that continued to manifest itself in this catastrophic fashion in Europe until the eighteenth century was bubonic plague.8 There is, however, no good reason for supposing these ancient epidemics were outbreaks of bubonic plague. Any of the familiar civilized infections—whether propagated via the respiratory tract, like measles, smallpox, and influenza, or via the alimentary canal, like typhoid and dysentery—could have produced the sort of sudden outbreak of mortality recorded in the Bible.9

  All one can properly conclude therefore is that diseases of this type were familiar to ancient Middle Eastern populations well before 500 B.C. and must have played roles of some importance in reducing population density from time to time and in affecting the course of military events. But the ravages of such diseases clearly were not enough to disrupt armies regularly nor to keep population below levels necessary for empire-building. Otherwise the Assyrian and Persian empires could not have flourished as they did between the ninth and fifth centuries B.C. It follows that epidemic diseases of the sort that attracted the attention of biblical writers were neither severe nor frequent enough to threaten the fabric of civilized society with disruption. In other words, from the point of view of the disease organisms, they were on the way to arriving at a mutually tolerable accommodation to their human hosts. Animal reservoirs (as with bubonic plague) may of course have played a role in allowing some infections to survive between epidemic outbreaks, but human populations of the ancient Middle East were certainly large enough to sustain the ancestors of modern childhood diseases on a fluctuating endemic basis.

  In a few major centers of population and communication, where human chains of infection had an optimal chance of becoming permanently established, some of these diseases were probably becoming common childhood afflictions according to the pattern familiar today. Epidemic outbreaks would then occur mainly in outlying regions, where population density was insufficient to sustain the infection on a long-lasting basis, but where unusual conditions (often connected with military operations) might trigger a sudden outbreak of infection, intense enough and sufficiently disastrous to human life to attract the attention of the learned priests and scribes who shaped the biblical texts in which such events are referred to.

  If these deductions are correct, civilized infectious diseases were only a little behind the diseases incident to irrigation agriculture in achieving a balance with their host populations in the ancient Middle East. As the locus of the oldest civilizations of the earth and one of the largest concentrations of human population in the world as of 500 B.C., the Middle East offered adequate time and opportunity for microparasitic as well as macroparasitic balances to approach stability within conditions defined by village and city life. More particularly: since the earliest surviving literary references to epidemic disease date back to about 2000 B.C., there had been sufficient time by 500 B.C. for some reasonably stable patterns of infectious diseases to establish themselves in the anciently civilized, much fought over, and densely populated regions of the Middle East.10

  By contrast, greater instability prevailed in fringe areas where three different natural environments—the Yellow River flood plain, the monsoon lands of the Ganges Valley, and the Mediterranean coastlantds—had all become capable of supporting civilized social structures much more recently than was the case in the Middle East. Accordingly, in 500 B.C. ecological balances were still precarious in these regions, and there is reason to suppose that disease patterns were far less firmly fixed than in the Middle East.

  Ecological instability can be attested in the first place by the comparatively massive population growth that was under way in each of these environments both before and after 500 B.C. Evidence is circumstantial but no less certain on that account. Without large-scale increase in human numbers the territorial expansion that each of these civilizations underwent would have been impossible. In each case, moreover, population growth was associated with far-reaching technical adjustments in patterns of agriculture and with appropriate elaboration of the respective macroparasitic political and cultural structures that gave each civilization an enduring and characteristic form throughout subsequent history.11

  In the Far East, Chinese peasants began to make real progress in farming the flood plain of the
Yellow River from about 600 B.C. This involved extending agricultural operations beyond the semi-arid environment of the loess soils where earlier Chinese agriculture had been at home, and shifting from millet to rice as the staple crop. A vast labor of diking, draining, canalization, and reclamation of swamp and marsh had to be carried through before the vast flood plain could be transformed into an almost unbroken carpet of rice paddies, each with a regulated access to water. In addition, the cultivated area as a whole had to be defended against dangers of flood and drought by an extensive and elaborate system of engineering works designed to control the tumultuous waters of the Yellow River.

  This stream is one of the most geologically active large rivers in the world. Recently (geologically speaking) it annexed important tributaries from other drainage systems, and in making its way through the loess country in its middle course, the river erodes vast quantities of soil, cutting its channel deeper every year. When silt-laden waters then debouch upon the almost flat flood plain, the current slows so that massive deposition succeeds the no less massive erosion upstream. As a result, in the flood plain the river builds up its bed rather rapidly. This made for trouble when men started to restrain the stream with artificial dikes. To be sure, the dikes could be built a little higher each year to match the deposition in the river’s bottom. But the result was that soon the great river began to flow seaward across the fertile flood plain above the level of the surrounding land. Enormous human effort was required to keep it there, since any runlet finding a pathway through a dike could quickly grow into a rushing torrent if not checked in time. Even a few hours might suffice to tear a gaping hole in the dike; and whenever massive gaps did occur, the entire river spilled out from its artificial bed, seeking a new, lower channel for itself. Several times the great river has thus shifted course by hundreds of miles, debouching either north (as at present) or south of the Shantung highlands.12

  The geological instability of the Yellow River was exacerbated but not created by human activity; and it will take geologic time spans for the river to achieve a more stable adjustment of its flow. Other dimensions of the ecological instabilities affecting early China were nearer the human scale. At the political level, for instance, the enlarged food resources produced by rice paddy cultivation sustained centuries of warfare among rival princes, until in 221 B.C. a single conqueror mastered the whole Yellow River flood plain as well as a broad band of adjacent territory both north and south of the river. After one brief bout of further civil war, a new dynasty, the Han, emerged to supremacy in 202 B.C., and remained in at least nominal control of all China until A.D. 221.

  Internal peace secured by an imperial bureaucratic administration probably diminished the costs to the peasantry inherent in earlier chronic warfare. Yet the Han peace also meant consolidation of a double layer of human macroparasites upon peasant rice (and millet) fields. Private landowners, who extracted rents, and official representatives of the Emperor, who extracted taxes, from the same peasant population obviously were in competition, yet they also supported each other most effectively. Their interests were basically the same, for in fact, the members of the imperial bureaucracy were recruited in large part from the landowning rentier class.

  There was, however, another powerful factor in the macroparasitic balance that began to define itself in ancient China. As Chinese landowners consolidated claims on the peasantry, a distinctive set of ideas and ideals of conduct also took root among the landlord and official classes. These are commonly called Confucian because the sage Confucius (traditional dates 551–479 B.C.) did a great deal to articulate and define the new ideals. The propagation of Confucian culture among imperial officials and private landowners internalized an ethic that strenuously restrained arbitrary or innovative use of power. One critically important consequence was to keep exactions imposed upon the peasantry within traditional and, under most circumstances, tolerable limits.

  As a result, by the time of the Emperor Wu-ti (140–87 B.C.), a remarkably stable and long-lasting balance was achieved within Chinese society between peasant farmers and the two social classes most directly parasitic upon them. This balance survived, with some important elaboration but no real structural breaks, until the twentieth century. Overall, we can be sure that the demands of landlord and official tax collector, heavy though they were, did not take more from the Chinese farmers than they were capable of producing over and above the minimum required for their own survival. Otherwise the slow, majestic march of the Chinese population throughout the Yellow River flood plain and adjacent regions, and then southward into and beyond the Yangtze Valley, could not have occurred; nor could the Chinese peasantry have offered a persistently expanding base (despite innumerable local and some general and long-lasting setbacks) for the imposing cultural and imperial structures of traditional China.

  Existing literature does not permit anyone to follow the pace of this Chinese advance with any exactitude. Yet massive development of the South did not occur until after the end of the Han Dynasty. In other words, almost a thousand years elapsed from the time when the taming of the Yellow River flood plain got seriously under way before comparable development took place in the valley of the Yangtze River.13

  At first glance this relatively slow pace of Chinese settlement in more southerly parts of what is today China may seem surprising. Political-military obstacles were relatively unimportant. Agricultural conditions favored settlement, since milder climates meant longer growing seasons, and more abundant rainfall removed the risk of drought that often endangered crops on unirrigated land in the North. Moreover, the fact that the Yangtze passes through lakes after it emerges from the mountains of the West means that no troublesome quantities of sediment clog its lower reaches. The awkward buildup of the river bed, characteristic of the Yellow River, was thus absent. Correspondingly, dikes and artificial networks for water distribution escape the extraordinary pressures they encounter in the North. The awesome, recurrent, and inescapable technical disasters that distinguished the history of the Yellow River valley simply do not occur.

  Despite these obvious and real advantages, an invisible and unrecorded but, one must still believe, very potent obstacle stood in the way of the swift and successful development of rice paddies and urban life in lands to the south of the historic cradle of Chinese civilization: for in moving southward and into better farming regions, Chinese pioneers were also climbing a rather steep disease gradient!

  The climatic shift involved is comparable to the difference between New England and Florida, but the lie of the land and prevailing wind patterns make the transition sharper than any climatic gradient occurring along the East Coast of North America. A mountain barrier shelters the Yangtze Valley from the cold and dry northwest winds that pour across the Yellow River valley from the Mongolian plateau in winter, constituting the winter monsoon. Correspondingly, in summer, when the monsoon winds blow the opposite way, warm, moist air sucked in from the South China Sea assures abundant precipitation in the Yangtze region. But these summer winds shed most of their moisture while crossing the mountain barrier before reaching the Yellow River valley, so that rainfall there is frequently insufficient to ward off damaging drought on unirrigated fields.

  The result is a sharp climatic difference between northern and central China. Among other things, the warmer, moister condition of the South allowed a greater variety of parasites to flourish than could survive in the North. Throughout the Yellow River flood plain, the severe winters killed off parasites that lacked dormant forms capable of resisting prolonged freezing. Important insect carriers of disease were similarly inhibited from establishing themselves because they could not survive the cold and dry conditions of the North. Nothing of the kind occurs in the Yangtze Valley south of the sheltering mountains. Populations accustomed to disease conditions of the North therefore faced formidable problems in adjusting to the markedly different patterns of parasitism that prevailed farther south.

  The earlier shift from dr
y farming on loess soils to irrigation farming in the Yellow River flood plain must also have exposed Chinese peasants to new and perhaps initially formidable disease risks. But whatever microparasitic adjustments occurred in connection with this change went hand in hand with far more conspicuous and time-consuming adjustments of a technical and macroparasitic kind. Centuries of effort were required to learn the arts of water management on a scale suitable for taming the Yellow River, and problems of political consolidation and modulation of human macroparasitism upon the peasantry were no less critical and time-consuming. Any adjustments to intensified disease risks could and did therefore occur simultaneously with these other more conspicuous transformations of Chinese society and techniques.

  Which process was the critical one? It is of course impossible to say for sure, but the macroparasitic side seems to have been the slower to come into balance. The reason for making such a judgment is that political-military stability did not come to China until the very end of the third century B.C. Before that time, organized violence mounted in intensity throughout the Warring States epoch of Chinese history (403–221 B.C.), climaxing with the conquest of the entire Chinese area by a still semi-barbarous state of Ch’in in 221 B.C. By the time the macroparasitic balances of ancient China attained a new imperial definition under the Han Dynasty (202 B.C.—A.D. 221) Chinese peasants already had four centuries of experience with the conditions of rice paddy farming behind them. Such a length of time gave ample opportunity for the epidemiological consequences of irrigation agriculture to stabilize themselves in the Yellow River valley, generations or even centuries before the macroparasitic side came into balance.