Plagues and Peoples

by William H. McNeill

  Trade, too, sometimes allowed the support of otherwise surplus population. Until recent centuries, however, the cost of transportation overland was so high that significant numbers of people could prosper through trade only by locating themselves near the sea or along navigable rivers. Nevertheless, from the earliest days of civilization ships could and did bring food and other useful commodities from afar to a number of ports. By exchanging manufactures and other goods for food and raw materials, civilized merchants and seamen could engage in mutually advantageous trade with foreigners. But it was as difficult to maintain trade balances in a steady state as it was to maintain a stable demographic balance within a single political community. Hence, sharply alternating expansion and contraction was the rule in trade as well as in politics and war.

  With such multiple instabilities built in, it seems clear that civilized society has not yet attained anything like a well-adjusted ecological balance on the macroparasitic level. Like a disease invading an inexperienced host population, the incidence of civilized forms of macroparasitism have fluctuated sharply through recorded history—sometimes killing off excessive numbers of the peasants and other workers who sustained the system by their labor, and at other times failing to hold the number of mouths at a figure to match available food.

  Despite innumerable local setbacks, however, the areas subject to civilized patterns of organization did tend to increase across the centuries. Yet the number of discrete civilizations always remained modest, though whether one counts a total of half a dozen or two dozen depends on the criteria used to distinguish one style of civilized life from another. Such small numbers reflect the fact that civilizations do not characteristically expand by stimulating the elaboration of pre-existing local institutions, ideas, and skills to new heights of sophistication. Instead, civilizations regularly export key cultural elements from an already elaborated center onto new ground. Often, perhaps always, it was easier to borrow and imitate than to create anew. There was, however, another factor in the situation that goes far to explain the comparative ease with which civilized societies expanded into new territories, one that was a result not of conscious policies or of macroparasitic patterns, but of the dynamics of microparasitism. A moment’s reflection will show what these were.

  When civilized societies learned to live with the “childhood diseases” that can only persist among large human populations, they acquired a very potent biological weapon. It came into play whenever new contacts with previously isolated, smaller human groups occurred. Civilized diseases when let loose among a population that lacked any prior exposure to the germ in question quickly assumed drastic proportions, killing off old and young alike instead of remaining a perhaps serious, but still tolerable, disease affecting small children.41

  The disruptive effect of such an epidemic is likely to be greater than the mere loss of life, severe as that may be. Often survivors are demoralized, and lose all faith in inherited custom and belief which had not prepared them for such a disaster. Sometimes new infections actually manifest their greatest virulence among young adults, owing, some doctors believe, to excessive vigor of this age-group’s antibody reactions to the invading disease organism.42 Population losses within the twenty-to-forty age bracket are obviously far more damaging to society at large than comparably numerous destruction of either the very young or the very old. Indeed, any community that loses a substantial percentage of its young adults in a single epidemic finds it hard to maintain itself materially and spiritually. When an initial exposure to one civilized infection is swiftly followed by similarly destructive exposure to others, the structural cohesion of the community is almost certain to collapse. In the early millennia of civilized history, the result was sporadically to create a fringe of half-empty land on the margins of civilized societies. Simple folk brought into contact with urban populations always risked demoralizing and destructive disease encounters. Survivors were often in no position to offer serious resistance to thoroughgoing incorporation into the civilized body politic.

  To be sure, warfare characteristically mingled with and masked this epidemiological process. Trade, which was imperfectly distinct from warlike raiding, was another normal way for civilized folk to probe new lands. And since war and trade relations have often entered civilized records, whereas epidemics among illiterate and helpless border folk have not, historians have hitherto failed to take anything like adequate notice of the biological weapon urban conditions of life implanted in the bloodstreams of civilized peoples. Absence of documentation should not, however, deter us from recognizing the force of the epidemiological superiority civilized conditions of life created among those who had survived the local mix of childhood diseases.

  Nonetheless, even when local populations had been decimated and demoralized by exposure to one or more diseases of civilization, effective obstacles to civilized encroachment on neighboring territory sometimes remained. If the land was too dry, too cold, too wet or too hilly for the agricultural methods familiar to the civilized community, then settlement was inhibited, and local peoples might have a chance to recover biologically, or be reinforced by some other population filtering in from more distant regions. If contacts became chronic between a civilized center and such a border zone, repeated exposure would deprive civilized diseases of most of their terrors. Occasional disasters might still occur in these borderlands if a new form of infection appeared, if human density increased to a point at which new patterns of disease propagation could sustain themselves, or if too long a period of time elapsed between exposures to forms of infection whose permanent hearth remained in the cities of civilization.

  But when no geographic or climatic barrier prevented the established methods of civilized cultivation from spreading into borderlands, peoples shattered by exposure to novel diseases were unlikely to be able to resist further encroachment. The process, in fact, resembles ordinary animal digestion rather closely. First, the structural organization of neighboring communities was broken down by a combination of war (cf. mastication) and disease (cf. the chemical and physical action of stomach and intestines). Sometimes, no doubt, a local population suffered total extinction, but this was not typical. More often, the shattering initial encounters with civilization left substantial numbers of culturally disoriented individuals on the land. Such human material could then be incorporated into the tissues of the enlarged civilization itself, either as individuals or as small family and village groupings. After mingling for a while with emigrants and refugees from the civilized interior, such populations became indistinguishable from other rural and remote elements of the civilized body politic. The way in which human digestion regularly breaks down the larger chemical structures of our food in order to permit molecules and atoms to enter into our own bodily structures seems closely parallel to this historical process.

  Observed from the civilized side of the frontier, an initial die-off and disruption of local social defenses opened the way for an overabundant civilized peasantry to move onto new ground and there find a fresh chance to thrive. For the most part this phenomenon remained sporadic and local. Suitable lands and surplus manpower were by no means always available. But it happened often enough across the centuries to allow recurrent bursts of expansion on the part of pre-existing civilized societies. In fact, it is fundamentally because of this phenomenon that civilized societies throughout history have so persistently tended to expand their geographical size.

  Of course, collisions between expanding civilizations occurred too, beginning in relatively early times, when Mesopotamian and Egyptian imperial governments started to clash in Syria and Palestine after about 1300 B.C. Moreover, the epidemiological and cultural “digestion” of one society by another has sometimes dissolved civilized communities as well. This was the fate of Amerindian civilizations after 1500. It happened also to ancient Egypt and Mesopotamia in the course of their gradual incorporation into imperial structures stretching beyond their original borders—a process completed only
after the Moslem conquests of the seventh century A.D.

  Some readers will boggle at this series of assertions and a priori deductions, especially when applied to civilized societies en bloc and without taking account of local differences and alterations across time. Undoubtedly there were such differences. But surviving records are incapable of discerning them, since the few who could write were completely unaware of the biological process I have tried, even if clumsily, to anatomize. We must reconcile ourselves to the fact that until modern times, when the phenomenon assumed unparalleled proportions as a result of European oceanic explorations that broke through innumerable epidemiological barriers, surviving records simply do not take notice of what happened to the weak and unfortunate neighbors of civilized peoples.

  Writers, naturally enough, tended to assume that the expansion of civilization (their own, of course) was only to be expected, since its charms and value were self-evident. Modern historians often unthinkingly assume the same thing. But given the normal attachment human beings feel for the ways of life to which they have been brought up, it is doubtful whether intact foreign communities ever opted for incorporation into an alien body social, even when the encroaching community possessed obvious and undeniable superiorities of skill, wealth and knowledge.

  To be sure, barbarians often enough triumphed as conquerors only to be conquered in their turn by the seductions of civilized ways. Such invaders probably seldom foresaw what would happen to their inherited life style, and they often struggled against civilized corruption when they finally began to recognize what was happening. Moreover, as conquerors and rulers, they always had far more attractive prospects than any available to poor and humble folk on the borderlands, whose appointed role was to be assimilated into the most oppressed class of civilized society. Such peoples may therefore be presumed to have always resisted incorporation into civilized society insofar as it lay within their power.

  If one tries to correct, therefore, for the built-in biases of available sources, the success civilizations so regularly demonstrated in incorporating border peoples into the fabric of metropolitan society needs explanation. Only if one gives appropriate weight to the epidemiological patterns described above does the expansion of civilized cultural frontiers become intelligible. Nothing else seems in the least adequate, or to accord with ordinary human behavior.

  For my argument India offers a sort of test case. In that subcontinent, a civilized level of society arose initially in the semi-arid Northwest, where the Indus River runs through increasingly desert lands from the high Himalayas to the sea. Such a landscape was similar to that of ancient Mesopotamia and Egypt, and the irrigation agriculture that supported Indus civilization was probably very like that of the two ancient Middle Eastern civilizations. The basic pattern of Indian history was defined by massive barbarian (Aryan) invasions after 1500 B.C., followed by a slow reassertion of civilized patterns of life. This, too, closely conforms to the rhythms of ancient history as experienced in the other river valley civilizations.43

  Divergence becomes unmistakable after about 800 B.C., however, when civilized social structures re-established themselves in northwestern India. These urban communities bordered to the south and east upon a landscape occupied by various “forest peoples” who lived, at least usually, in small, self-contained communities of a sort that in temperate zones were extremely vulnerable to epidemiological undoing by civilized diseases. There is no reason to think that civilized diseases were not just as disruptive in India as they were in more northerly parts of Eurasia. But the forest peoples in India did not crumple up and disintegrate as might have been expected. Instead, they had their own epidemiological riposte to the biological armament of civilization. Various tropical diseases and parasitic infestations that flourished in moist and warm climates protected them against the temperate zone pattern of civilized encroachment. As was true later in Africa, death and debility lurked in too many forms to allow massive or rapid invasion of moist, warm regions by civilized personnel from India’s drier North and West. A sort of epidemiological standoff ensued. Forest folk might be decimated by infections arising from contacts with civilized peoples, but civilized intruders were equally vulnerable to contacts with the tropical diseases and infestations familiar among the forest folk.

  The upshot is well known. Instead of digesting the various primitive communities that had occupied southern and eastern India in the manner that was normal north of the Himalayas, Indian civilization expanded by incorporating ex-forest folk as castes, fitting them into the Hindu confederation of cultures as semi-autonomous, functioning entities. Local cultural and social traditions were therefore not destroyed before being fitted into Indian civilized social structures. Instead, a vast variety of primitive rites and practices survived for centuries. Every so often such elements surfaced within the Indian literate record, when orally transmitted ideas and rituals attracted the attention of literate individuals and were duly written down, elaborated or distorted so as to fit into the pullulating complexity of historic Hinduism.

  Other elements and attitudes of course entered into the definition and maintenance of the caste principle in Indian society. Yet the taboos on personal contact across caste lines, and the elaborate rules for bodily purification in case of inadvertent infringement of such taboos, suggest the importance fear of disease probably had in defining a safe distance between the various social groups that became the castes of historic Indian society. Only after a prolonged process of epidemiological encounter, during which antibody immunities and tolerances of parasitic infestation were gradually equalized (or initial differences sharply reduced) did it become safe for Aryan-speaking intruders to live side by side with speakers of Tamil and other ancient tongues. Genetic blending (despite caste rules against intermarriage) no doubt accompanied this epidemiological exchange, and a fairly rigorous selective survival must have altered gene assortments among the forest peoples as well as among intrusive representatives of civilized styles of life.

  Yet all such homogenizing processes fell short of the drastic “digestive” pattern characteristic of the other Old World civilizations. Consequently, the cultural uniformity and sociological cohesion of the Indian peoples has remained relatively weak in comparison to the more unitary structures characteristic of the northerly civilizations of Eurasia. One may, of course, attribute this peculiarity of the Indian style of civilization to chance or to conscious choices. Chance and choice may indeed have played a role in defining the caste principle; but the unique epidemiological situation confronting Indian civilization in its early phases of expansion must also have a great deal to do with making castes what they became, thereby defining the structure of Indian civilized society in a different way than prevailed elsewhere.

  The situation in the Americas was different in another way. Civilized diseases of the kind that arose in the major Eurasian centers of urban life failed to establish themselves before 1500 in Mexico and Peru. Otherwise Montezuma would surely have had a more efficacious epidemiological revenge on the invading Spaniards than in fact manifested itself. It seems best, however, to reserve a more careful consideration of American disease patterns until a later chapter, when the epidemiological consequences of European arrival in America will become the subject of our consideration.

  Here it remains to summarize the results of all these inferences and arguments based on modern notions of infectious disease. Despite the lack of conclusive literary or archaeological evidence, it seems sure that the major civilized regions of the Old World each developed its own peculiar mix of infectious, person-to-person diseases between the time when cities first arose and about 500 B.C. Water-borne, insect-borne, and skin-to-skin infections also had a much expanded scope within the crowded cities and adjacent regions of dense agricultural settlement. Such diseased and disease-resistant civilized populations were biologically dangerous to neighbors unaccustomed to so formidable an array of infections. This fact made territorial expansion for civilized population
s much easier than would otherwise have been the case.

  Exact boundaries between the different disease pools cannot be ascertained. No doubt the geographic range of any particular infection varied from year to year, depending on movement of people, fluctuations of virulence, and patterns of incidence within the civilized centers themselves. The result was acutely unstable. The novel biological balances—both micro- and macroparasitic—which civilized social structures had created were liable to further disturbance with every significant alteration of transport and communication, since none of the important new infections had reached geographical or other natural limits. Exploration of how these balances altered in the period from 500 B.C. to A.D. 1200 will be the theme of the next chapter.

  III

  Confluence of the Civilized Disease Pools of Eurasia: 500 B.C. to A.D. 1200

  B

  y 500 B.C. different micro- and macroparasitic balances had established themselves in each civilized region of Eurasia, and unstable accommodations between human hosts and the new civilized diseases had begun to manifest themselves in some and probably in all of the major civilized centers.

  Exact definition of disease balances is altogether impossible, even for the oldest and best-known of these centers, in the Middle East. Here the original irrigation core had been supplemented after about 2000 B.C. by the establishment of cities and organized states on rain-watered land. Civilized patterns of social organization thereafter became endemic wherever good agricultural soil was to be found. A broad belt of civilized lands therefore arose on both the eastern and western flanks of Mesopotamia; a more slender fringe also expanded Egyptian influence into both eastern and northern Africa.

  The ebb and flow of empires these circumstances permitted is well known. Akkadian, Babylonian, Kassite, Mittanian, Hittite, Egyptian, Assyrian, Chaldean, and Persian conquerors succeeded one another amid tumultuous war and recurrent influxes of barbarians from the borderlands. Successive imperial structures tended to grow ever larger and better organized, expanding toward natural limits set by those conditions of soil and climate that restricted peasant agriculture. With the establishment of the Persian empire in the sixth century B.C., these limits were approximately reached. By 500 B.C., the borders of that empire—on the north, south, and east—abutted on steppe and desert lands where prevailing methods of cultivation would not have brought in lush enough crops to bear the cost of an expanded imperial administration.