Plagues and Peoples

by William H. McNeill

  Similar uncertainty prevails in countries like Thailand, where HIV-1 infections are believed to be widespread. Worldwide, uncertainty is even greater, but so far at least, AIDS simply does not compare with the ravages of the Black Death or with the disease destruction of Amerindian and other isolated peoples in the more recent past.

  A really efficacious medical remedy for AIDS would alter the situation, of course; but so far chemical treatments only prolong sufferers’ lives without definitively curing the infection. Meanwhile, only partially understood changes in the HIV-1 virus itself are presumably taking place—partly in response to chemical attacks and partly in response to encounters among diverse strains coming from different parts of the world.

  Modern air travel assures continuation of this accelerated process of disease homogenization, not merely for HIV-1 but for all other human, animal, and plant infections as well. It means that whenever a new, especially successful form of an infection emerges, it will spread rapidly around the globe. The pattern of the flu virus, evolving a new variant almost every year, is archetypical. Other, usually unidentified viruses afflict us constantly as well. It is obvious that human (and nonhuman) diseases are evolving with unusual rapidity simply because changes in our behavior facilitate cross-fertilization of different strains of germs as never before, while an unending flow of new medicines (and pesticides) also present infectious organisms with rigorous, changing challenges to their survival.

  AIDS therefore is not the only infection to threaten human lives; but overall and in the long run, the familiar ecological pattern of mutual accommodation between host and parasite is almost sure to prevail. That means endemic will tend to replace epidemic infections while less lethal strains and chronic infections gain in importance against more virulent germs.

  Plants and animals share with us in the process of disease homogenization. Probably, local wild-animal populations are most at risk simply because they have not previously encountered some of the infections that speedy, intensified global transportation exposes them to. Other changes in habitat, due largely or entirely to human activity, also affect wild plant and animal populations. Overall, the result is massive extinctions of isolated species, with long-range consequences we cannot anticipate.

  In short, organic evolution is operating in high gear nowadays, largely because of human intervention in natural ecosystems. Our exposure to infectious disease is altering very quickly, but this is only part of a wider adjustment and readjustment of ecological relations whose future trajectory remains as mysterious as ever. Perhaps one can say that biological evolution has, in effect, been overtaken and accelerated beyond all precedent by human intervention in natural processes, guided and directed by modern science on the one hand, and driven by multiplying human numbers on the other.

  In a sense, this is as old as humanity itself, for even our remotest predecessors also altered their environments by deliberate, concerted action, directed by conscious intentions and defined by shared meanings that were expressed mainly through words. But modern science and technology, by tapping inorganic sources of energy, enormously enlarged human capabilities for changing natural balances among competing life forms. The apparent conquest of infectious diseases between 1884, when Robert Koch first identified the cholera bacillus, and 1976, when WHO succeeded in eliminating smallpox, was assuredly one of the most drastic disturbances of older ecological balances ever achieved by human beings. Nonetheless, the way infectious diseases have begun to come back shows that we remain caught in the web of life—permanently and irretrievably—no matter how clever we are at altering what we do not like, or how successful we become at displacing other species.

  This book explores one important aspect of our extraordinary capability for altering natural balances, and the limitations of those capabilities. Nothing that has happened since it was written contradicts its general thrust. We remain part of the earth’s ecosystem, and participate in the food chain whereby we kill and eat various plants and animals, while our bodies provide a fair field full of food for a great variety of parasites. No conceivable change in the earth’s ecosystem will alter that fundamental condition of human life, even though changes in our knowledge and behavior can and will continue to alter the incidence of disease and the array of what we eat.

  This book explores the notable landmarks of the shifting balance between human hosts and infectious organisms. It is a dramatic story whose importance for political and cultural history is only beginning to be widely understood. I therefore invite you to read what I wrote more than twenty years ago and see for yourselves how infectious disease affected our predecessors’ lives.

  W. H. McN.

  15 March 1997

  * Mirko D. Grmek, History of AIDS: Emergence and Origins of a Modern Pandemic (Princeton, New Jersey, 1990). French edition, Histoire de SIDA (Paris, 1989). I owe most of what I understand about AIDS to this book and to a more recent summary, Gabriel Rotello, Sexual Ecology: AIDS and the Destiny of Gay Men (New York, 1997).

  * Rotello, op. cit., pp. 73-74.

  Introduction

  HOW THIS BOOK HAPPENED

  N

  early twenty years ago, as part of my self-education for writing The Rise of the West: A History of the Human Community, I was reading about the Spanish conquest of Mexico. As everyone knows, Hernando Cortez, starting off with fewer than six hundred men, conquered the Aztec empire, whose subjects numbered millions. How could such a tiny handful prevail? How indeed? All the familiar explanations seemed inadequate. If Montezuma and his friends first thought the Spaniards were gods, experience soon showed otherwise. If horses and gunpowder were amazing and terrible on first encounter, armed clashes soon revealed the limitations of horseflesh and of the very primitive guns the Spaniards had at their disposal. Cortez’s skill in finding allies among the Indian peoples of Mexico and rallying them against the Aztecs was certainly important, but most of his Indian allies committed themselves to the Spanish side only when they had reason to think Cortez would win.

  The extraordinary story of the conquest of Mexico (soon to be followed by Pizarro’s no less amazing conquest of the Inca empire in South America) was really only part of a larger puzzle. Relatively few Spaniards ever were able to cross the ocean to the New World, yet they succeeded in impressing their culture on an enormously larger number of Amerindians. The inherent attraction of European civilization and some undeniable technical superiorities the Spaniards had at their command do not seem enough to explain wholesale apostasy from older Indian patterns of life and belief. Why, for instance, did the old religions of Mexico and Peru disappear so utterly? Why did villagers not remain loyal to deities and rituals that had brought fertility to their fields from time immemorial? The exhortation of Christian missionaries and the intrinsic appeal of Christian faith and worship seem insufficient to explain what happened, even though, in the eyes of the missionaries themselves, the truth of Christianity was so evident that their success in converting millions of Indians to the faith seemed to need no explanation.

  A casual remark in one of the accounts of Cortez’s conquest—I can no longer tell where I saw it—suggested an answer to such questions, and my new hypothesis gathered plausibility and significance as I mulled it over and reflected on its implications afterward. For four months after the Aztecs had driven Cortez and his men from their city, an epidemic of smallpox broke out among them, and the man who had organized the attack on Cortez was among those who died. Such an epidemic, striking an entirely inexperienced population, was dreadful in itself, and no one knew how to respond or what to do. Since the population lacked inherited or acquired resistances, something like a quarter to a third of them presumably died from the initial onslaught.

  Moreover, it is worth considering the psychological implications of a disease that killed only Indians and left Spaniards unharmed. Such partiality could only be explained supernaturally, and there could be no doubt about which side of the struggle enjoyed divine favor. The religions, pri
esthoods, and way of life built around the old Indian gods could not survive such a demonstration of the superior power of the God the Spaniards worshiped. Little wonder, then, that the Indians accepted Christianity and submitted to Spanish control so meekly. God had shown Himself on their side, and each new outbreak of infectious disease imported from Europe (and soon from Africa as well) renewed the lesson.

  The lopsided impact of infectious disease upon Amerindian populations therefore offered a key to understanding the ease of the Spanish conquest of America—not only militarily, but culturally as well. But the hypothesis swiftly raised other questions. How and when did the Spaniards acquire the disease experience that served them so well in the New World? Why did the Amerindians not have diseases of their own with which to mow down the invading Spaniards? Tentative answers to such questions soon began to uncover a dimension of the past that historians have not hitherto recognized: the history of humanity’s encounters with infectious diseases, and the far-reaching consequences that ensued whenever contacts across disease boundaries allowed a new infection to invade a population that lacked any acquired immunity to its ravages.

  Looked at in this way, world history offered a number of parallels to what happened in the Americas in the sixteenth and seventeenth centuries. This book describes the main lines of these fateful encounters. My conclusions will startle many readers, since events but little noticed in traditional histories assume central importance for my account. This is because the long line of learned scholars whose work it was to sift surviving records from the past has not been sensitive to the possibility of important changes in disease patterns.

  To be sure, a couple of spectacular examples of what can happen when an unfamiliar infection attacks a population for the first time have never been expunged from European memory. The Black Death of the fourteenth century was the chief example of this phenomenon, and the cholera epidemics of the nineteenth century constitute a second, far less destructive, but more recent and better-documented instance. Historians, however, never saw these as belonging to a more general class of critically important epidemiological breakthroughs because earlier examples of disastrous encounters with new diseases lay buried deeper in the past, where records were so imperfect that both the scale and the significance of what happened were easy to overlook.

  In appraising ancient texts, historians were naturally governed by their own experience of epidemic infection. Living amid disease-experienced populations, where relatively high levels of immunity to familiar infections damped any ordinary epidemic outbreak very quickly, critically trained historians were impelled to discount as exaggeration any remark about massive die-off from infectious disease. Failure to understand the profound difference between the outbreak of a familiar disease amid an experienced population and the ravages of the same infection when loosed upon a community lacking acquired immunities is, indeed, at the bottom of the failure of previous historians to give adequate attention to the whole subject. Assuming that infections had always been present in much the same fashion as they were in Europe before the advent of modern medicine, there seemed nothing much to say about epidemics, and historians tended, therefore, to pass such matters by with only the sort of casual mention I found in the account of Cortez’s victory.

  History of epidemics became the province of antiquarians, who took pleasure in recording essentially meaningless data simply because it was there. Yet there remained the Black Death, together with a number of instances when a sudden outbreak of disease in an army abruptly altered military circumstances, and sometimes determined the outcome of a campaign. Such episodes could not be left out, but their unpredictability made most historians uncomfortable. We all want human experience to make sense, and historians cater to this universal demand by emphasizing elements in the past that are calculable, definable, and, often, controllable as well. Epidemic disease, when it did become decisive in peace or in war, ran counter to the effort to make the past intelligible. Historians consequently played such episodes down.

  To be sure, there were a number of outsiders, like the bacteriologist Hans Zinsser, who played devil’s advocate, picking out instances when disease did make a difference. Thus Zinsser’s eminently readable book, Rats, Lice and History, showed how outbreaks of typhus often upset the best-laid plans of kings and captains. But such books did not try to fit disease experience into any larger picture of human history. For them as for others, occasional disastrous outbreaks of infectious disease remained sudden and unpredictable interruptions of the norm, essentially beyond historical explanation and therefore of little interest to serious professional historians whose job it was to explain the past.

  This book aims to bring the history of infectious disease into the realm of historical explanation by showing how varying patterns of disease circulation have affected human affairs in ancient as well as in modern times. Many of my suggestions and inferences remain tentative. Careful examination of ancient texts by experts in many different and difficult languages will be needed to confirm and correct what I have to say. Such scholarly work requires a thesis to test, a target to shoot down. The speculation and guesswork I have indulged in ought to serve this purpose, and in the meantime, it can draw ordinary readers’ attention to important gaps in older ideas about the human past.

  Quite apart from details of what I have to say, everyone can surely agree that a fuller comprehension of humanity’s ever-changing place in the balance of nature ought to be part of our understanding of history, and no one can doubt that the role of infectious diseases in the natural balance has been and remains of key importance.

  A FEW KEY CONCEPTS

  Before proceeding with the story, a few remarks about parasitism, disease, pestilential infection, and related concepts may help to avoid confusion.

  Disease and parasitism play a pervasive role in all life. A successful search for food on the part of one organism becomes for its host a nasty infection or disease. All animals depend on other living things for food, and human beings are no exception. Problems of finding food and the changing ways human communities have done so are familiar enough in economic histories. The problems of avoiding becoming food for some other organism are less familiar, largely because from very early times human beings ceased to have much to fear from large-bodied animal predators like lions or wolves. Nevertheless, one can properly think of most human lives as caught in a precarious equilibrium between the microparasitism of disease organisms and the macroparasitism of large-bodied predators, chief among which have been other human beings.

  Microparasites are tiny organisms—viruses, bacteria, or multi-celled creatures as the case may be—that find a source of food in human tissues suitable for sustaining their own vital processes. Some microparasites provoke acute disease and either kill their host after only a brief period of time, or provoke immunity reactions inside his body that kill them off instead. Sometimes, too, one of these disease-causing organisms is somehow contained within a particular host’s body so that he becomes a carrier, capable of infecting someone else without being noticeably sick himself. There are, however, other microparasites that regularly achieve more stable relations with their human hosts. Such infections no doubt take something away from their host’s bodily energies, but their presence does not prevent normal functioning.

  Macroparasites exhibit similar diversity. Some kill at once, as lions and wolves must do when feeding on human or any other kind of flesh; others allow the host to survive indefinitely.

  In very early times, the skill and formidability of human hunters outclassed rival predators. Humanity thus emerged at the very top of the food chain, with little risk of being eaten by predatory animals any more. Yet for a long time thereafter cannibalism almost certainly remained a significant aspect of the interaction of adjacent human communities. This put the successful human hunters exactly on a level with a pride of lions or a pack of wolves.

  Later, when food production became a way of life for some human communities, a modu
lated macroparasitism became possible. A conqueror could seize food from those who produced it, and by consuming it himself become a parasite of a new sort on those who did the work. In specially fertile landscapes it even proved possible to establish a comparatively stable pattern of this sort of macroparasitism among human beings. Early civilizations, in fact, were built upon the possibility of taking only a part of the harvest from subjected communities, leaving enough behind to allow the plundered community to survive indefinitely, year after year. In the early stages the macroparasitic basis of civilization remained harsh and clear; only later and by slow degrees did reciprocal services between town and countryside develop importance enough to diminish the one-sidedness of tax and rent collection. To begin with, though, the hard-pressed peasantries that supported priests and kings and their urban hangers-on received little or nothing in return for the food they gave up, except for a somewhat uncertain protection from other, more ruthless and shortsighted plunderers.

  The reciprocity between food and parasite that has under-girded civilized history is matched by parallel reciprocities within each human body. The white corpuscles, which constitute a principal element in defenses against infection, actually digest intruders. Organisms they are unable to digest become parasites, digesting in their turn whatever they find nourishing within the human body.

  This is, however, only one facet of the exceedingly complex processes that affect the success or failure of any particular organism in invading and proliferating within a particular human being. In fact, despite all the advances of medical research in the past hundred years or so, no one fully understands their interaction. At every level of organization—molecular, cellular, organismic, and social—one confronts equilibrium patterns.2 Within such equilibria, any alteration from “outside” tends to provoke compensatory changes throughout the systems so as to minimize over-all upheaval, though there are always critical limits which, if transgressed, result in the breakdown of the previously existing system. Such a catastrophic event may involve dissolution into simpler, smaller parts, each with equilibrium patterns of its own; or, on the contrary, may involve incorporation of smaller parts into some larger or more complex whole. The two processes may in fact combine, as in the familiar case of animal digestion, whereby the feeder breaks down the cells and proteins of its food into simpler parts only to combine them into new proteins and the cells of its own body.