Plagues and Peoples

McNeill states:

…optimal conditions for host and parasite occur, often though not necessarily always, when each can continue to live in the other’s presence for an indefinite period of time with no very significant diminution of normal activity on either side (28).

Disease is therefore not defined by the mere presence of the bubonic plague in a human host, for instance, but by its maladaptation to it. The corresponding adaptation is usually a matter of hard-won resilience or quarantine.

These imbalances are caused in two ways. First, in epidemic situations, a new microparasite can be introduced to a virgin host. Should the host become the only food source, eventually the parasite will adapt. The interim result is disease. In other cases, diseases can become endemic to places humans live, but will have adapted themselves to some other life form, only incidentally to kill the occasional, accidental human host for which there is no need to develop an adaptive appetite. Such recurrence in humans is also called disease. 

The property of being endemic is that of being localized among a discrete group or locality. In epidemiology, endemic diseases are localized and long-standing afflictions. For instance, modern malaria is endemic to parts of Africa and South America, but not elsewhere.

In many cases, microparasites endemic to places humans live adapt to them as a food source, and in some cases, form a symbiosis. Human stomachs are endemic locales for thousands of gut parasites that perform a beneficial aid to digestion as provided a food source. For instance, anywhere human beings breathe on one another, particularly in tightly packed indoor spaces, become endemic sites of influenza transmission. Every year, human bodies become an escalating battleground of viral parasites versus protective antibodies, and this, too, describes an endemic relationship.

In Greek, epidemic comes from the root words for epi (upon) and demos (the people), but from the time of Hippocrates, the term has come to stand as a synonym for the spread of often deadly disease. McNeill uses the term in common with other researchers as a way of actively describing spreading disease, into places where it was not before present. When smallpox first hit the shores of the Americas—a land that had never seen smallpox—it became one of the worst epidemics in recorded history.

In this way, epidemics are defined by their novelty and have been given special attention by past historians interested in notable and outlying events like war and drought. The fall of Rome was bookended by sudden outbreaks of measles during the Antonine Plague, and of the bubonic plague during the Justinian Plague. Historians give these special attention because their suddenness and drama produced decisive pivots in culture and leadership style. The cost of this attention, argues McNeill, is that it ignores the infrastructural effect of endemic disease, which is less dramatic but arguably more impactful over time.

When considering microparasites, they are rarely considered as individual units, or even as individual cultures located within one body. To the scientific researcher, it would hardly be worth it to consider the effect of one individuated influenza virus from any others. Rather, successful tracking of disease is done over generations of disease, through round upon round of balancing antibody testing. An epidemiologist’s definition of a microparasite, by contrast, would likely begin with a discussion of any visible parasite, such as a leech or a tapeworm. Yet the way the term “macroparasite” is used in Plagues and Peoples is unique to McNeill, serving to apply this same scientific disinterest to human society and finding metaphoric shared ground between the growth of human society and disease.

“A conqueror could seize food from those who produced it, and by consuming it himself became a parasite of a new sort on those who did the work,” McNeill writes in his Introduction (25). Since recorded history, human beings have been alpha predators at the top of their respective food chains. This makes parasitism their only real threat and in the world of the visible, this parasitism happens between human beings. Such a relationship nakedly describes despotism, and McNeill makes little distinction between democratic and authoritarian rule. Rather, like its small cousin, human parasitism is defined by its stability and balance. Thus, the long period after the foundation of Confucianism marked a stability and order allowing for profound growth in Chinese demography; the disorder of Roman paganism, by contrast, offered little in the way of redemption for personal suffering and was a less sustainable way to implore an already sick and battered Christian peasantry to keep feeding their pagan landlords. McNeill treats the clever particularities of these forms of rule as incidental.