Plagues and Peoples

In the original 1976 Introduction to his book, McNeill outlines the scope and limitations of his study. He defines parasites in terms of those that can be seen and those that cannot—“macroparasites” and “microparasites.”

McNeill begins his first chapter deep within human pre-history:

Before fully human populations evolved, we must suppose that like other animals our ancestors fitted into an elaborate, self-regulating ecological balance. The most conspicuous aspect of this balance was the food chain, whereby our forebears preyed upon some form of life and were, in their turn, preyed upon by others (35).

In this, he speculates what early hominid primates’ relationship to the environment was like, and places humans within a continuum which is not separate from the living things humans eat and that eat humans.

Tracing humankind’s earliest relationship to disease gives an incomplete picture. Fossil records found in sub-Saharan Africa do little to trace the spread of disease among early peoples. McNeill speculates as to the spread of disease from observing modern-day primates in those same places. Sub-Saharan Africa, writes McNeill, is host “to an impressive roster of protozoa, fungi and bacteria, not to mention more than 150 so-called arbo-viruses” which are conveyed from host to host (36). This number far exceeds the microscopic life found in colder and drier climates.

This does not mean that primates in past- or present-day Africa are perpetually sick. McNeill states, “tropical rain forests support a highly evolved natural balance at every level” (37). It would not do, for instance, for the myriad gut bacteria populating the stomachs of all mammals to start killing those on which they depend to survive. Just as humankind is thought to have come out of the wetlands and begun populating drier savannahs in search for food, so too do microorganisms adapt to survive. Sometimes these changes are useful but evolutionarily sudden in bodies adapting to new evolutionary niches.

In this context, disease is what occurs when such useful niche alterations are not produced. For instance, the microorganism that produces sleeping sickness is well adapted to the antelope and to the tsetse fly who feeds on the antelope’s blood; when introduced to either animal’s bloodstream, it does not cause sickness. However, when the tsetse fly feeds on a human host, causing the microorganism to be transferred to a body in which it has not adapted, it does cause sickness. This sickness is good neither for the host or the parasite, yet it is a mere side effect of the beneficial relationship the microorganism has to the original host, and therefore only slowly corrected by evolutionary progression.

McNeill metaphorically compares microflora and macroflora, noting that the virulence and rapidity of humankind’s spread across the globe in just a few hundred millennia could be likened to a disease, forcing hasty ecological rebalancing; he further suggests disease acts as a natural limitation and rebalancing for such rapid growth, while admitting such interactions are “extremely complex” (44).

Evidence of Homo sapiens, or fully modern human physiology, was scattered with contradictory archeological signs leading in and out of Africa over the course of 50,000 to 100,000 years ago. The invention of tools and clothing increased the spread of humanity, and McNeill surmises “health and vigor improved” as humanity surged into colder and drier climates (48). The opposite was true for the macrofauna human beings discovered as they migrated: “Indeed, one calculation suggests that skilled and wasteful human hunters took a mere thousand years to exterminate most large-bodied game in North and South America” (48).

McNeill concludes with the theory that while humanity had an intimate survival relationship to microflora in the tropics, as it entered into colder and drier climates, its survival became focused on large-bodied animals, and upon the observation of landscape characteristics which made effective, coordinated Paleolithic hunting techniques possible. All of this changed around 20,000 BCE when massive climate change occurred as a result of retreating ice caps.

McNeill cites the retreating polar ice caps—circa roughly 11,000 years ago—and the massive die-off of large hunting animals leading up that time, as reasons for fundamental changes to human behavior in the years to follow, particularly as concerns the gathering of food and the development of societal orders.

Preexisting methods of food production, such as fishing and the soaking of cassava and olives, were joined during this time to an explosion in human domestication of plants and animals. Though details of early domestication remain unclear, the evidence points to rapid and extensive changes within the scale of time indicated. McNeill marks this period by six important changes to the human record: 1) the increase of domesticated plant and animal species, 2) the shortening of food supplies, reducing biological diversity, 3) a new focus on protecting cultivated lands from human and non-human parasitism, 4) an increase in regular patterns of work, particularly in regard to land cultivation, 5) increased incidences of flooding, in order to reroute water to cultivable lands, and finally, 6) the creation of increasingly complex tools used to work the land. McNeill says:

One must admire the skill with which humankind discovered and exploited the possibilities inherent in remodeling natural landscapes in these radical ways, increasing human food supply many times over, even though it meant permanent enslavement to an unending rhythm of work (58).

During this period, humankind developed innovations such as crop rotation, rice paddy farming, and animal-powered plowing.

Chief among farmers’ concerns were the infestation of parasites in visible forms such as weeds, weevils, rats and mice, as well as in hidden forms that spread disease. As abundant and limited crops grew, so did the parasitic forms attached to them, in a process called hyperinfestation. In a similar process, civilization in the form of villages and cities bred new hyperinfestations of disease among human beings, aided by increased contact with disease-bearing animals and humans, still water, and collected human waste.

Civilization did not shrink from increased exposure to disease-bearing parasites; in fact, it tremendously grew over the course of millennia. Nevertheless, disease profoundly shaped human society, and while human society has radically changed from an evolutionary perspective, humans today are not much physiologically different from their ancestors. The same is true of the parasites existing today. By observing how disease affects human contemporaries McNeill postulates how disease must have affected prehistoric communities.

Schistosomiasis, which McNeill uses as an example of many diseases, and which is spread to humans by water-borne parasitic flatworm, creates a “listless and debilitated” peasantry (63). This affected the peasantry’s ability to protect themselves from other threats—particularly that of parasitic humankind. Thus, many cultural and religious traditions—such as the ancient Jewish and Moslem prohibition of pork or the expulsion of lepers from society—came to pass in order to protect human life. By the same token, due to ancient misunderstandings about the ways microparasites transfer from host to host, traditions such as communal bathing helped foster rather than halt the spread of disease.

The spread of human society and of concomitant human-on-human predation was often halted by an imbalance of disease resistance. People living in the rainforests of West Africa were better accustomed to malarial strains that would kill invaders from drier climates. By the same token, civilized people cultivated fragile “childhood” diseases which appear only when human beings appear en masse. These diseases, such as chicken pox, would follow resistant and asymptomatic city dwellers into less populous regions, where they would devastate the peasantry. Cohabitation with domesticated animals—such as cows and pigs—as well as “pest” animals—like rats and monkeys—also spread new forms of disease, such as cowpox, influenza, bubonic plague, and measles, among many others.

Sometimes these diseases would wipe out whole populations of people or animals in a single generation, though in most cases these are only noted in historical records. McNeill instead focuses on the steady historical progression of disease as it affected war and instability within the limited ancient historical record. McNeill uses a digestive metaphor, in which neighboring communities are first “broken down by a combination of war (cf. mastication) and disease (cf. the chemical and physical action of stomach and intestines)” before incorporating the new mass of peoples into the health of the budding empire (88). He uses India as an example, in which the balance of diseases between the civilized and warm-climate tropical peoples led to the creation of a caste system in which people at various level of disease could cohabitate within the same empire (or so McNeill deduces from the historical record).