Plagues and Peoples

by William H. McNeill

  By way of comparison, the nadir of the rabbit population in Australia occurred in 1953, three years after the initial outbreak of myxomatosis. Given the brevity of rabbit generations—observed as six to ten months from birth to parenthood in Australia—this three-year span was equivalent to 90 to 150 years on a human scale, if we calculate a human generation to be 25 years.34 In other words, comparable generational time may be needed for humans and for rabbits to adjust to an initially lethal new disease.

  The entire process of adjustment between host and parasite may be conceived as a series of wavelike disturbances to preexisting biological equilibria. The initial disturbance is likely to be drastic, as happened among Australian rabbits in 1950. In many cases, transfer of parasitism to a new host species is too drastic to persist very long. Assuming, however, that the new infection is able to survive indefinitely, a fluctuating balance then asserts itself, with periods of unusual frequency of infection alternating with periods when the disease wanes and may almost disappear. These fluctuations tend to stabilize themselves into more or less regular cycles—that is, as long as some new major intrusion from “outside” does not alter the emerging equilibrium pattern between host and parasite. A multitude of factors enters into any such cyclic equilibrium. Seasonal changes in temperatures and moisture, for instance, tend to concentrate childhood diseases in modern cities of the temperate zone in spring months.

  The number of susceptible persons in a population is also fundamental, as are the ways in which they congregate or remain apart. School and military service, for example, have been the two most significant ways susceptible youth congregate in modern times. Any parent can attest the role of primary schools in propagating childhood diseases in contemporary western societies: in the nineteenth century, before inoculations became standard, draftees into the French army from the countryside suffered—sometimes seriously—from infectious disease to which their city-bred contemporaries were almost immune, having already been exposed. As a result, robust peasant sons had a far higher death rate in the army than did undernourished weaklings drafted from urban slums.35

  The size of dose required to infect a new host, the length of time during which the infection may be transferred from one person to another, modes of such transfer, and customs affecting opportunities for exchanging infections, all play roles in determining how many get sick and when. Not infrequently a disease requires a massive, megalopolitan concentration of human hosts to survive indefinitely. In such a population the chance of encountering enough susceptible new hosts so as to keep a chain of infection going is obviously greater than when potential hosts are scattered thinly across a rural landscape. Yet when enough susceptible persons exist in rural communities, such a disease can sally forth from its urban focus and run like a terrifying brushfire from village to village, household to household. Such outbreaks, however, fade away as rapidly as they arise. As the local supply of susceptible hosts runs out, the infection dies and disappears, except in the urban center whence it had initially emerged. There, enough susceptibles will remain for the infectious organism to keep itself alive until disease-inexperienced individuals again accumulate in the rural landscape and another epidemic flare-up becomes a possibility.

  All these complex factors sometimes shake down to relatively simple over-all patterns of incidence. Careful statistical study of the way measles propagates itself in modern urban communities shows a wave pattern cresting in periods of time just under two years. Moreover, it has recently been demonstrated that to keep this pattern going, measles requires a population with at least 7,000 susceptible individuals perpetually in its ranks. Given modern birth rates, urban patterns of life and the custom of sending children to school, where measles can spread very rapidly through a class of youngsters meeting the virus for the first time, it turns out that the minimal population needed to keep measles going in a modern city is about half a million. By scattering out across a rural landscape, a smaller population suffices to sustain the chain of measles infection. The critical threshold below which the virus cannot survive falls between 300,000 and 400,000 persons. This can be demonstrated by the way the measles infection behaves among island populations ranking above and below this critical mass.36

  No other disease current in our own time exhibits so precise a pattern, and none, probably, requires such large human communities for its survival. Comparably exact studies for other common childhood diseases have not been made, largely because artificial immunization procedures have altered patterns of infection in far-reaching ways in all modern countries. Yet notable changes in virulence as well as in the frequency of the most common childhood diseases have occurred as recently as the nineteenth century, when European governments first started to collect statistics on the incidence of separate infectious diseases. In other words, the adjustment between the disease-causing organisms and their human hosts was (and is) still evolving very rapidly, in response to the altering circumstances and conditions of human life.

  Searching historic records for evidence of when and where the ancestors of our modern childhood diseases first invaded human populations can be quite frustrating. First of all, ancient medical terminology cannot easily be fitted to modern classifications of disease. Symptoms alter, and undoubtedly have altered, so much as to be unrecognizable. At first onset, a new disease often exhibits symptoms that later disappear when the host population in question has had time to develop resistance.

  The fulminating symptoms that syphilis initially exhibited in Europe is a familiar example of this phenomenon from the past. Similar episodes can be observed today whenever a new disease invades a previously isolated community for the first time. Symptoms can, indeed, be such that they completely disguise the nature of the disease from all but expert bacteriological analysis. Thus, for example, when tuberculosis first arrived among a tribe of Canadian Indians, the infection attacked organs of their bodies which remained unaffected among whites. Symptoms—meningitis and the like—were far more dramatic, and the progress of the disease was far more rapid, than anything associated with tuberculosis infections among previously exposed populations. In its initial manifestations, only microscopic analysis allowed doctors to recognize the disease as tuberculous. By the third generation, however, the tuberculosis infection tended to concentrate in the lungs, as mutual accommodation between hosts and parasites began to approximate the familiar urban pattern.37

  The process of adaptation between host and parasite is so rapid and changeable that we must assume that patterns of infection prevailing now are only the current manifestations of diseases that have in fact altered their behavior in far-reaching ways during historic times. Yet in view of the figure of half a million needed to keep measles in circulation in modern urban communities, it is noteworthy that a recent estimate of the total population of the seat of the world’s oldest civilization in ancient Sumeria comes out to exactly the same figure.38 It seems safe to assume that the Sumerian cities were in close enough contact with one another to constitute a single disease pool; and if so, massed numbers, approaching half a million, surely constituted a population capable of sustaining infectious chains like those of modern childhood diseases. In subsequent centuries, as other parts of the world also became the seats of urban civilizations, ongoing infectious chains became possible elsewhere. First here, then there, one or another disease organism presumably invaded available human hosts and made good its lodgment in the niche increasing human density had opened for it.

  Person to person, “civilized” types of infectious disease could not have established themselves much before 3000 B.C. When they did get going, however, different infections established themselves among different civilized communities in Eurasia. Proof of this fact is that when communications between previously isolated civilized communities became regular and organized, just before and after the Christian era, devastating infections soon spread from one civilization to another, with consequences for human life analogous to, though less drastic than, what
happened to rabbits in Australia after 1950.

  Closer consideration of these events will be reserved for the next chapter. Here it seems only needful to reflect briefly about the general historical consequences of the establishment of these distinctively civilized sorts of diseases in a few centers of unusually dense human population between 3000 and 500 B.C.

  First and most obvious: patterns of human reproduction had to adjust to the systematic loss of population that resulted from exposure to diseases that flourished under civilized conditions. Until very recently cities were unable to maintain their numbers without a substantial inflow of migrants from surrounding countrysides. Urban health hazards were simply too great, for, in addition to infectious person-to-person diseases transmitted as childhood diseases usually are—by breathing in droplets of infectious matter sneezed or coughed into the atmosphere—ancient cities suffered from an intensified circulation of diseases transmitted through contaminated water supplies, plus a full array of insect-borne infections. Any breakdown of transportation bringing food from afar threatened famine, and local crop failures were often difficult to compensate for. In view of all this it is not surprising that cities could not maintain themselves demographically, but had to depend on migrants from the countryside to replenish the losses arising from famine, epidemic, and endemic diseases.

  A civilized pattern of life therefore required rural cultivators not only to produce more food than they themselves consumed in order to feed urban dwellers, but also to produce a surplus of children whose migration into town was needed to sustain urban numbers. Rural reproductive surpluses had also to be capable of bearing losses resulting from macroparasitism, i.e., from war and raiding, and from the famine such activities nearly always provoked. Only occasionally and for limited periods of time was anything like a stable balance attained between rural birth rates and occupational niches available in urban contexts for the surplus from the countryside. Open and accessible frontiers—so important for European history in the past four centuries—were also unusual, though when land was available, surplus rural population could and did migrate to the frontier and thereby enlarge the agricultural base of the society instead of trying the risky (though to a few, spectacularly rewarding) path of migration into town.

  Until after 1650, when population statistics begin to assume a degree of reliability, it seems impossible even to guess at the magnitudes involved in this pattern of population flow. Nevertheless, such patterns clearly asserted themselves from the time cities first formed. The striking way, for example, in which Sumerian-speakers gave way to Semitic-speakers in ancient Mesopotamia during the third millennium B.C.39 is probably a direct consequence of this kind of population movement. Speakers of Semitic tongues presumably migrated into Sumerian cities in such numbers that they swamped speakers of the older language. Sumerian lingered on as a language of learning and priestcraft, but for everyday purposes, the Semitic Akkadian took over. This linguistic shift might have resulted from a spurt of urban growth, or more likely from an unusually heavy die-off of established urban populations because of disease, war or famine, although which of these factors or combination of factors may have been at work in ancient Sumer is not known.

  A nineteenth-century parallel may be useful. From the 1830s and especially after 1850, rapid urban growth together with the ravages of a new disease, cholera, disrupted cultural patterns of long standing in the Hapsburg monarchy.40 Peasant migrants into the towns of Bohemia and Hungary had long been accustomed to learn German, and in a few generations their descendants became German in sentiment as well as in language. This process began to falter in the nineteenth century. When the number of Slav- and Magyar-speaking migrants living in the cities of the monarchy passed a certain point, newcomers no longer had to learn German for everyday life. Presently nationalist ideals took root and made a German identity seem unpatriotic. The result was that Prague became a Czech- and Budapest a Magyar-speaking city within half a century.

  Early civilizations that were linguistically more uniform obviously did not register the process of migration into town by linguistic change as ancient Mesopotamia and the nineteenth-century Hapsburg monarchy did. Nonetheless, the reality of urban population wastage in very ancient as well as in more recent times cannot be doubted. The mere existence of cities and the intensified patterns of disease circulation they created must have led to this result, with only as much delay as was needed for disease organisms to discover and work themselves into the enriched environment urbanized humanity presented for their nutriment.

  How the flow of surplus population from the countryside was provoked and sustained is not at all clear. To be sure, the country was often healthier, since various forms of infection rife in cities were less likely to reach rural dwellers. On the other hand, when an epidemic did penetrate the countryside it could have more drastic consequences than were likely among an already diseased and therefore partially immune urban population. Moreover, many peasantries were chronically undernourished and therefore especially vulnerable to any infection that happened along. Clearly, peasantries subjected to civilized control did not automatically find it easy to raise more children than were needed to keep the family operation going, any more than they found it easy to produce more food than they themselves required for survival.

  Yet universally they accomplished both of these tasks. Civilizations could not have persisted without a flow of migrants as well as a flow of food from countryside to city. It is, therefore, altogether probable that moral codes encouraging a high rural birth rate were a necessary underpinning for civilized patterns of society. At any rate, the various means by which hunting and gathering communities regulated their numbers have not prevailed among civilized peasantries. Instead, in most if not all peasant societies, early marriage and a long string of children has been regarded as a sign of moral excellence and divine favor, as well as the best of all possible assurances against a helpless old age, since if one child should die another can still take on the responsibility for looking after the old folks when they are no longer able to support themselves. These attitudes were also connected with recognition of individual and familial property rights to land. Such rights were often, in turn, defined or reinforced by governmental policy with respect to rent and taxes.

  Exactly how cultural, social, and biological factors acted and reacted upon each other, however, is impossible to tell. All we can be sure of is that successful civilizations all managed to assure a flow of persons as well as of goods from the countryside into the cities, and they did so through combining the sanctions of religion, law and custom.

  As will be readily appreciated in our age of explosive population growth, the civilized reproductive norm ran the risk of provoking acute rural overpopulation. Any prolonged slackening of career opportunities for the peasant surplus—in cities, armies, or by emigration to some frontier region—soon had the effect of ponding excess population back in villages. To forestall rural overpopulation, alternative careers had to involve high death rates, yet without deterring large numbers of men and women from accepting the risks involved, whether they did so voluntarily or involuntarily, knowingly or in ignorance, of the probable upshot of leaving home.

  To keep a stable demographic balance under such circumstances was and is exceedingly difficult. Urban and military die-off must match rural growth rates, and the whole community must simultaneously succeed in defending itself against “outside” invasion so massive as to upset its internal demographic pattern.

  A genuinely stable macroparasitic pattern conforming to these specifications has rarely existed for long in any part of the world. Instead, civilized history has characteristically exhibited sharp fluctuations up and down, as periods of peace and prosperity induced population growth in excess of macroparasitic powers of absorption (i.e., destruction); whereupon an increase in death rates asserted itself through the breakdown of public order. Peasant revolt, civil war, foreign raid and rapine, together with accompanying intensification
of famine and disease, could always be counted on to reduce populations catastrophically whenever less drastic regulators of peasant numbers failed to maintain a satisfactory balance.

  Characteristically, heightened death rates would cut back peasant numbers far below previous levels before successful political consolidation would again allow rural population growth to assert itself. Obviously, “outside” invasions—whether by disease organisms or by armed men—were capable of interrupting such cycles; so could unusual climatic conditions that resulted in heavy crop losses. Indeed, in most of the civilized world such “outside” factors were so powerful and so frequent as to mask any close correlation between the oscillation of peasant numbers and the level of public peace. Only in China, where external political-military forces were weaker because geographic barriers insulated the civilized human mass from important foreign pressures most of the time, did this cycle manifest itself unmistakably; though even there, extraneous factors were never entirely absent and sometimes held back population recovery for centuries at a time.

  Civilized societies had another way of consuming surplus population from the countryside. By mounting attacks on neighboring regions, kings and armies were sometimes able to expand the territories under their control and open frontier lands for their subjects to settle and exploit. Such enterprises, indeed, offered an all but infallible solution to any danger of overpopulation at home, since a notable increase in the number of deaths could always be expected from wars of conquest, whether they were successful or not.