Plagues and Peoples

“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.”

McNeill sets out the dual nature of his book, which is the story of the biomechanical process of humanity’s growth and the parasitical gates to that spread. Much of the richness of humanity’s spread found in other historians’ narratives are lost here; war, politics, and culture come down to simple parasitism in McNeill’s ecological perspective.

“If the alternate host is somehow more important to the parasite, adaptation toward a stable biological balance will concentrate on adjustment to its nonhuman host.” 

This is the process by which a disease goes from being epidemic to endemic within a group of people. On the other hand, diseases with only incidentally transfer to humans from more suitable hosts, such as the bubonic plague from rats to humans, never undergo this process of endemicity. 

“Although statistical and clinical data allowing precise definitions of which infections killed how many people, when and in what places are unattainable before the nineteenth century—and remain spotty even then—we may still observe major changes in patterns of pestilential infection. This, in fact, is the subject of this book.” 

When they exist, historic records on disease written before the 19th century are spotty and unreliable. Here, McNeill admits that much of the work he does to piece together his work is performed through speculation deriving from the behaviors of diseases as they have existed since the introduction of legible scientific taxonomy.

“Incidentally, it is not absurd to class the ecological role of humankind in its relationship to other life forms as a disease.” 

McNeill acknowledges the rising awareness of humankind’s place on earth as not just an observing, persevering and heroic narrative subject, but as the object of much larger forces it tends to ignore.

“The general thinning out of the variety of life forms that took place as climates became colder and/or drier implies, after all, a diminution in the number and variety of parasitic organisms capable of afflicting human beings.”

The receding of the polar ice caps fundamentally altered humankind’s relationship to the landscape. Colder and drier climates are host to fewer microparasites, opening territories previously locked off from humanity.

“But toil—persistent, unending and fundamentally at odds with humankind’s propensities as shaped by the hunting experience—was nevertheless the lot of all farming populations.” 

The rise of agriculture and the reduction of human food consumption around massive land-roaming animals altered human beings’ relationship to itself. Nevertheless, it is important to note that “round-the-clock toil” is a propensity of industrial societies. The toil in farming societies is much more seasonal and cyclical than is suggested here.

“Shortening the food chain and multiplying a restricted number of domesticated species of plants and animals also created dense concentrations of potential food for parasites.” 

The collecting together of biomass in the form of humans, livestock, and harvest agriculture concentrated the microbiota that relied upon them, while also restricting the sort of ecologies keeping that microbiota in check. Though unrecorded, we can speculate that the early millennia of agricultural discovery and production must have been fertile grounds for epidemic disasters. 

“A mere five thousand years ago, therefore, parasitic forms of life exploiting the special conditions created by irrigation agriculture were probably almost identical with those that still make like difficult for modern irrigation and rice paddy farmers.” 

Rice paddy farming is among the oldest forms of agricultural production, and the most problematic from an epidemiological point of view. Mosquitos and water-borne diseases are endemic to the shallow waters rice paddy farmers stand in for many hours a day.

“Lassitude and chronic malaise, in other words, of the kind induced by blood fluke and similar parasitic infections, conduces to successful invasion by the only kind of large-bodied predators human beings have to fear; their own kind, armed and organized for war and conquest.” 

In other words, human conquerors soon learn they have nothing to gain from the conquered dead. Instead, much of human parasitism is the effort of conquerors to keep the conquered in a state fit enough to work and produce value. In this, they mirror the activities of microparasites.

“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 populations much easier than would otherwise have been the case.” 

Often, the promulgation of new epidemics was caused by massed and civilized people entering dispersed human settlements, bringing with them the endemic diseases to which they had earned resistance. By the same token, however, the outskirts of civilization were sites of new disease civilized people had not yet encountered.

“In other words, almost a thousand years elapsed from the time when the taming of the Yellow River flood plain got seriously underway before comparable development took place in the valley of the Yangtze River.” 

The more southerly Yangtze was difficult to civilize, compared to the safer and less microparasitic Yellow river. What was required to populate the Yangtse, then, was an unusually long-lasting balance in macroparasitic forces (or what other historians would call political peace and coordination). The Confucian set of ethics, which restrained the arbitrary use of power, made such balance possible.

“Definite population estimates are possible only for the Roman world and for Han China.” 

Again, these are only estimates taken from less than demographically accurate sources. Here, McNeill emphasizes not the wealth of material from which he is working, but the paucity of it, considering its enormous scope.

“We may infer that by about the beginning of the Christian era, at least four divergent civilized disease pools had come into existence, each sustaining infections that could be lethal if let loose among populations lacking any prior exposure or accumulated activity.” 

These “civilized disease pools” represent the flourishing cultures of the Mediterranean, Southern and Northern Chinese, and Indian civilizations. As travel infrastructure and technology increased in the form of caravansaries and improved ship design, these civilizations would begin to overlap, spreading new epidemics.

“An infectious disease which immunizes those who survive, and which returns to a given community at intervals of five to ten years, automatically becomes a childhood disease.” 

The general trend of endemic disease is that its effects become less severe as human and microparasite adjust in accordance to an ecological balance. What this means, in human terms, is that there are a whole host of endemic diseases that tend to affect the weakest members of a civilization—children and the elderly—while sparing those more productive members who do the work of either being a parasite or feeding parasites in the macroparasitic order.

“Not only did large numbers of persons travel very long distances across cultural and epidemiological frontiers; they also traversed a more northerly route than had ever been intensively traveled before.” 

Parasitism could only restrain human civilization by so much. McNeill points out that the evidence of this is indisputably underlined by humans’ modern presence and profile on the planet; humankind has grown at a steady pace from its inception to the modern day, transforming ecologies everywhere it goes.

“What probably happened between 1331 and 1346 therefore, was that as plague spread from caravanserai to caravanserai across Asia and eastern Europe, and moved thence into adjacent human cities wherever they existed, a parallel movement into underground rodent “cities” of the grasslands also occurred.” 

McNeill’s argument, original to this book, is that the bubonic plague spread not through a sudden accident whereby shipbound merchants brought infected rats with them by port, but through an infrastructure that affected the very ecology of Eurasia. It is an argument that takes seriously ecology, rather than as an afterthought to human progress.

“In face of intense and immediate crisis, when an outbreak of plague implanted fear of imminent death in an entire community, ordinary routines and customary restraints regularly broke down.” 

McNeill spends very little time on civilizational culture but does note that macroparasitic balance tends to break down during times of crisis, exacerbating other problems. In this quite conservative reading, the breakdown of “ordinary customs” is usually indicative of societal collapse, rather than of its strengthening.

“By 1568, less than fifty years from the time Cortez inaugurated epidemiological as well as other exchanges between Amerindian and European populations, the population of central Mexico had shrunk to about three million, i.e., to about one tenth of what had been there when Cortez landed.” 

There is little record of what happened when the earliest “civilized disease pools” first encountered one another within Eurasian geographies. There are, however, richly written eyewitness accounts of what happened when the first Europeans landed in the Americas. The result was genocidal.

“On the time scale of world history, indeed, we should view the ‘domestication’ of epidemic disease that occurred between 1300 and 1700 as a fundamental breakthrough, directly resulting from the two great transportation revolutions of that age—one by land, initiated by the Mongols, and one by sea, initiated by Europeans.” 

Despite the explosion in reported incidences of disease (due in part to more record keeping as a matter of technological advance and cultural norms), the trend toward rapid expansion of demographic numbers indicates a general trend, and at least in Eurasia, toward the homogenization of disease pools. Deadly epidemics became less frequent, while endemicity of disease increased.

“The fundamental character of the changing incidence of epidemic disease was obscured in early modern times by the onset of particularly severe weather conditions that created frequent crop failures and famines in northern Europe.” 

History before McNeill describes medieval Europe as a hotbed of disease and death. McNeill says in endemic terms, the bubonic plague represents a global trend toward more survivable disease. However, medieval Europe was also defined by frequent agricultural blight and other macroparasitic problems obscuring the nature of medieval disease.

“With every fundamental of medicine thus called into question, the only logical recourse was to observe results of cures administered in accordance with the old Galenic as against the new Paracelsian theories, and then to choose whichever worked better.” 

The modern medical relationship to disease echoes the modern scholarly relationship to historiography, with practitioners trending toward a self-analysis of heroic human progress. Nevertheless, enormous material progress in medicine was made once adherence to outdated medical models was dropped.

“Before the findings of the astronomers and mathematicians of the seventeenth century could become a basis for a popularized world view, therefore, epidemic disease had also to relax its dominion over human minds and bodies.” 

Here, McNeill makes a big claim: Namely, the enlightenment may not have been possible without a steady increase in endemicity within the civilized disease pools of Eurasia. This claim centers Europe as the center of learning, sidelining a (more epidemically challenged) millennia of Chinese and Middle Eastern scientific and cultural progress.

“But from the 1880s onward, a series of dramatic triumphs accrued to medical researchers who succeeded in isolating and studying the ‘germs’ of one infectious disease after another.” 

Among the greatest breakthroughs of medical science was the advancement of the germ theory of disease, made concrete by the late-19th century improvement in magnification technology. After the microscope, one vaccine after another was invented to tame a host of diseases once dogging mankind.