Epidemic Man

Most professional historians resemble small peasants, intensively cultivating tiny tracts of the past; their labor is arduous and indispensable, but if they pause to raise their heads their vision is usually bounded by their neighbor’s fence. Some, bolder spirits, are like air travelers; as they peer down from thirty thousand feet, larger outlines of the past become visible; whole countries and centuries can be discerned, though sometimes the detail is rather blurred. Only a tiny few, powered by their own exceptional energy or the assistance of research foundations, project themselves above the earth altogether; for them the whole globe swims into focus and millennia of human history take shape before their eyes.

One of these rare historical astronauts is William H. McNeill, professor of history at Chicago. As the author of The Rise of the West, he is best known for his panoramic views of world history, though occasionally his spacecraft drops down nearer the earth’s surface so that he may give closer examination to a thousand years of Venetian history (Venice: The Hinge of Europe), or, skimming dangerously near the ground, a mere three hundred years of Europe’s Steppe Frontier. He has traveled a long way since his beginnings as a mere cultivator in 1947 with a PhD thesis on The Influence of the Potato in Irish History.

In Plagues and Peoples Professor McNeill returns to orbit; and, once again, has from his elevated vantage point been able to discern patterns which have hitherto eluded workers on the ground. He is not the first to study the history of disease; on the contrary, his work is wholly dependent on the labors of generations of medical historians and epidemiologists. But by reflective reading and imaginative analysis he has been able to give fresh shape and meaning to the subject. Founded on secondary sources though it is, Plagues and Peoples is a learned, cogent, and wholly absorbing book. It can be truly said that, to most of those who read it, the history of the world will never seem quite the same again.

Professor McNeill’s subject is the impact of infectious disease upon the course of human affairs. In part, this involves pointing out the historical effect of particular epidemics, like, for example, that of 430-429 BC, when a quarter of the Athenian land army was wiped out, so that Athens became incapable of defeating Sparta and the Peloponnesian League. But the book’s underlying aim is much more ambitious than this, for what McNeill seeks to do is to establish the major patterns in the history of infection itself. To this end he adopts an uncompromisingly biological standpoint, from which human history becomes a mere dimension of natural history. To understand the diseases which have affected humanity it is necessary to recall that man is himself a predator and that his impact upon other forms of life closely resembles that of an acute epidemic disease. The hunters of Paleolithic times, in a catastrophic overkill, managed to wipe out the large game animals on which they preyed, thus eliminating some two hundred distinct herbivores and dependent carnivores. Comparable devastation on plant and animal life has been wrought subsequently by peaceful agriculturalists. For McNeill the growth of human population is the result of an ecological upset, rather like the multiplication of rabbits in nineteenth-century Australia.

The survival of the human race, he argues, depends on the maintenance of a precarious equilibrium between the microparasitism of the invisible disease organisms which invade the human body and the macroparasitism of larger predators, whether wild animals or other human beings in their role as warriors, plunderers, and tax-gatherers. This uneasy equilibrium will be upset if the microorganisms cause their human hosts to die of infection or if the large-bodied predators make excessive demands upon the food producers on whom they prey. Men can defend themselves against macroparasites by erecting states and armies to repel foreign invaders and repress internal banditry; but, since the very existence of microparasites was unknown until the invention of the microscope, defense against them has, until recently, been largely a matter of unconscious biological adjustment.

It is this biological adjustment, involving an endlessly changing relationship between man and his microparasites, that is the main theme of Plagues and Peoples. The treatment of macroparasitism is less original and occupies less space, perhaps because its effects have usually been much less deadly than those of microparasites; McNeill points out that in every European war before the twentieth century deaths from disease far exceeded deaths from weapons. In any case, there is something rather strained about his concept of macroparasitism, for it implies that the depredations of war and politics should be regarded as biological facts, rather than the work of self-conscious human beings. McNeill’s penchant for biological analogy even leads him to suggest that the growth of modern industrial cities is “but the most recent and complex example of the processes whereby millions of atoms are regularly assembled into the larger organic molecules.” This could almost be the voice of Aristotle, save that even Aristotle recognized that the analogy between biological and social life is only an analogy: man is a thinking and reasoning creature whose behavior can only be metaphorically compared to that of unthinking plants or atoms.

On the relationship between man and his microparasites, however, McNeill is intensely interesting. Drawing on the findings of modern epidemiologists, he explains how this relationship can begin in a dramatic way, with one or both parties failing to survive the initial encounter. Any human population will be highly vulnerable when first exposed to an infection from which it has previously been isolated. A violent epidemic will ensue and mortality will be high. But the survivors and their children will often develop a resistance which permits humans and microorganisms to coexist peacefully. The disease will then return in waves of diminishing virulence. If the population is large enough to sustain a continuing chain of infection, the infection will ultimately decline to the status of a childhood illness, like measles today. (The recent history of myxomatosis among rabbits provides a dramatically speeded-up illustration of this process; in eight years the death rate shrank from an initial 99.8 percent to 25 percent; by contrast, a human population would have taken 120 to 150 years to stabilize its response to such a drastic new infection.)

Not all infectious diseases settle down in this way. Those which are primarily adapted to nonhuman hosts, like the bubonic plague carried by rodents, will remain lethal indefinitely; others, like influenza, confer only a brief period of immunity; new strains of virus permit them to return in a more virulent form, as in the great epidemic of 1918-1919, which killed over twenty million people. But usually the paradox is that “the more diseased a community, the less destructive its epidemics become.”

This constantly evolving process of adjustment between human hosts and disease-causing parasites is not a purely biological matter, but is greatly affected by changes in the pattern of social life. McNeill suggests that there are three particularly crucial ways in which men have by their own behavior unwittingly altered the pattern of disease. The first is by agricultural innovation. Sometimes this has had disastrous consequences, as when the development of Eastern irrigation disseminated the blood fluke which causes the debilitating disease of schistosomiasis, from which perhaps 100 million people suffer today. At other times the results have been beneficial: the spread of enclosures helped to segregate men from disease-carrying animals, while the plough reduced the plague-bearing rodents of the Russian steppe.

The second determining social fact has been the changing density of population. A disease like measles needs a population of nearly half a million before it can sustain itself. For this reason the migrant hunters of Paleolithic times seem to have been free from infectious diseases, or at least from those carried by microorganisms specifically adapted to the human species. Only when the rise of agriculture made possible the growth of cities did chains of continuing infection emerge. By 500 BC, McNeill thinks, each of the major civilized regions of the world had developed its own distinctive “mix” of infectious disease. Until the twentieth century cities remained peculiarly vulnerable to infections, particularly those carried by air or water, and their populations needed constant replenishment from the adjacent countryside.

The third and most critical social fact was the growth of human communications. The most original and important part of McNeill’s book is his powerful analysis of the resulting interaction between the inhabitants of hitherto distinct disease areas. He reminds us how the Amerindian inhabitants of the New World succumbed dramatically when exposed to the smallpox, measles, and influenza brought with them by their Spanish invaders; and he argues plausibly that malaria and yellow fever were also unknown in America before the coming of the Europeans. A civilized people which had learned to live with its childhood diseases thus acquired a potent biological weapon when it came into contact with a hitherto unexposed population. During the sixteenth century the original populations of Mexico and Peru were reduced by disease to less than a tenth of their original size. Cortez, with his six hundred troops, could never have conquered an Aztec empire of millions but for this staggering demographic collapse, accompanied as it was by the total demoralization of the Indians in face of disease to which their conquerors appeared bafflingly immune.

Less familiar to most readers will be McNeill’s demonstration that a very similar disaster had struck the Europeans themselves at the beginning of the Christian era. The overland trade between Asia and the Mediterranean which grew up from the second century AD brought with it first smallpox (?165-180 AD), then measles (?251-266), finally bubonic plague (542-543). Each had a devastating effect upon the defenseless Europeans, who lacked any resistance to these infections. If the Spanish conquest of America is inexplicable without taking into account the impact of infectious disease, the same can be said for the decline of the Roman Empire; and Professor McNeill does not hesitate to say it.

In his book McNeill traces the successive epidemical results of the growing interaction between civilizations. Each extension of world travel and trade involved not just cultural and economic contact but also the spread of some new infection, with devastating consequences for hitherto unexposed populations. The trade contacts initiated in Roman times brought deadly epidemics to China no less than to Europe. The Mongol horsemen transmitted plague to the rodents of the Eurasian steppe, thereby making possible the coming to Europe of the Black Death. The voyages of exploration upset the microparasitic balance in Europe as well as in America, for there appeared “new” diseases like typhus, syphilis (in its venereal form), and the “English sweat.”

Ultimately, the growth of world communications brought about not just a world market but also a world pool of disease, in which the established human infections circulated frequently enough among civilized populations to lose their original virulence. By about 1700 or 1750, McNeill thinks, a new, stable parasitism had been achieved. The process was not complete, for cholera and smallpox still had lethal achievements ahead of them. But the two great transport revolutions, initiated by the Mongols on land and the Europeans on sea, had “domesticated” epidemic disease. It was “a fundamental landmark in human ecology that deserves more attention than it has ordinarily received.”

The final stage in the reduction of epidemic disease is seen by McNeill mainly as the achievement of modern medicine, both therapeutic and preventive. He gives a fascinating account of the successive campaigns against smallpox, cholera, plague, malaria, and yellow fever, many of them stimulated less by humanitarian motives than by military or imperialistic ambitions. It must be said, however, that McNeill’s assessment of the modern medical achievement is a good deal more optimistic than that offered in a penetrating recent study by a British professor of social medicine, Thomas McKeown.¹ The latter agrees that the colossal rise of world population (from 750 million in 1750 to 4 billion in 1975) is to be largely attributed to the decline of infectious disease. But McKeown is reluctant to attribute this decline to medical science. Vaccination for smallpox apart, he is skeptical about the value of most forms of medical immunization, which he considers to offer briefer and less effective protection than experience of the disease itself; and he points out that deaths from tuberculosis, bronchitis, pneumonia, scarlet fever, measles, and whooping cough were all declining before the development of effective medical therapy.

McKeown also refuses to attribute the improvement to a changed relationship between microorganisms and their human hosts, save in the cases of scarlet fever, influenza, and, possibly, measles. For he thinks it incredible that a chronologically coincidental change should have diminished the virulence of all the other infections; and he points out that there is no evidence for the exceptionally high mortality from smallpox or the food- and water-borne diseases which would have been necessary in the eighteenth century to produce a genetically determined resistance thereafter. McKeown regards higher standards of hygiene and public health as the reason for the decline of food-and water-borne infections, but otherwise lays his main emphasis on improved nutrition, which he thinks stiffened human resistance to infection, a possibility to which McNeill gives only a passing mention.

Such disagreements about the causes of fluctuations in epidemic disease are unlikely ever to be convincingly resolved. Baffling enough in modern times, when the evidence is abundant, the topic becomes impossibly difficult for remoter periods. As McNeill wryly concedes, “exact information is lacking wherewith to create a history of human infections.” Confident identification of ancient diseases on the basis of contemporary descriptions is impossible, not least because the diseases themselves can disconcertingly change their symptoms over the centuries.

As for the decline of epidemics in modern times, it seems that at least five distinct causes have to be balanced against each other: 1) immunity acquired by previous exposure to the infection, 2) a decline in the natural virulence of the disease itself, 3) increased resistance through better nutrition, 4) diminished exposure to infection, through quarantine and better hygiene, 5) improved medical therapy. The respective importance of each of these causes will vary considerably according to the disease and the people concerned. In the circumstances, McNeill’s essay must be regarded as a pioneering effort, an ambitious interpretative synthesis of much hitherto unrelated work by historians and epidemiologists. It gives epidemic disease a much more central place on the historical stage than it has hitherto occupied. But, as McNeill is the first to admit, many of his conclusions inevitably remain speculative. It will take years of back-breaking labor by the historical peasantry before all his hypotheses are confirmed or rejected.

Meanwhile McNeill has triumphantly vindicated the central importance of his subject. He shows that there are few major events in human history which have not been affected, often decisively, by the prevailing pattern of disease. He ingeniously invokes the history of epidemics to explain not just the growth of population and the distribution of human settlement but also such diverse phenomena as the decline of Rome, the rise of Buddhism and Christianity, the expansion of Europe, the decline of Spain, the caste system of India, and the growth of the British Empire. On these subjects his arguments are usually persuasive and often convincing. Only in the field of ideas is his touch less certain. For he seems to regard morality, religion, and politics as merely biological “adjustments” to ecological needs. Thus the Hindu emphasis on bodily purification represents an intuitive fear of disease; the Jewish dietary code was meant to reduce infection; eighteenth-century deism was only possible because of the decline in Europe of plague, malaria, and smallpox; the Manchurian taboo on trapping marmots, because they might be their ancestors reincarnated, was an intuitive recognition of their plague-carrying propensities.

There are hints here of old-fashioned medical materialism; it is surprising not to find Professor McNeill arguing that the Roman Catholics opposed the extension of the chalice to the laity at the time of the Reformation because they “intuitively” recognized that infection would be more rapidly transmitted when everyone drank out of the same cup.² Fortunately, he is too good a historian not to point out that many religious rituals had epidemiologically disastrous consequences: pilgrimages spread infection; ceremonial bathing transmitted cholera; and when churchmen from all over the British Isles went home after the Synod of Whitby in 664 AD they took what became a country-wide epidemic with them. As a defense mechanism against microparasitism, religious ritual and belief have had their limitations.

The very scale of Professor McNeill’s endeavor means that his analysis is sometimes tantalizingly brief or superficial. There are many well-known points of dispute in the history of disease on which experts will find him disappointingly unhelpful; they may even regard his large-scale essay in synthesis as itself a form of macroparasitism upon their own narrower inquiries. But by showing so clearly that the incidence of infectious diseases is not random but the product of identifiable historical forces, McNeill has made a real contribution to the understanding of history. His book exudes intellectual power and a rare grasp of the essentials.