Disease and Commerce

In his well-known study Plagues and Peoples, and in subsequent writings as well, the historian William H. McNeill has shown that a useful way to understand the evolution of human society is to examine the complex interactions between and among micro-parasites and macroparasites. Microparasites, in this context, are the microscopic organisms that live off human tissue, sometimes carrying disease and death, sometimes provoking immune reactions in their host that destroy the microparasites, and sometimes developing a reciprocal relationship that allows both host and parasite to survive in a state of relative equilibrium. In the last case, the host may become a carrier of infection, capable of spreading it to others although he or she remains symptomless.

Macroparasites, in the traditional sense, are predatory animals such as lions and wolves that feed on the flesh of other animals - but McNeill suggests that, in a more metaphorical sense, humans who seize the goods of others or who compel others to provide services are also macroparasites. Like micro-parasites, macroparasites of this sort sometimes kill their hosts in the immediate assault, but more often they develop a long-term exploitative relationship, again a state of relative equilibrium, albeit to the much greater advantage of the parasite than the host (McNeill 1976, 1980).

Throughout the course of human evolution there have been long stretches of time when it seems that states of equilibrium have existed among the masses of people and the microparasites and macroparasites that lived off their bodies and their labor. To be sure, during these times neither the physical nor the social conditions of the majority of humankind may have been ideal, but neither were they often in a state of crisis. This was true even in the early centuries of urban society discussed earlier: Although certainly punctuated from time to time by epidemic or political upheaval - usually in tandem, the case of

Athens in the fifth century B.C. being the classic example (Shrewsbury 1950)-those early civilizations remained sufficiently separate from one another that biological and social stability was more the rule than the exception.

This began to change at about the time of the Christian Era (to use the Western chronological guidepost) as overland caravans and open-sea ships expanded trade throughout the Middle East, Asia, and Europe. The outer extremes of that new network of commerce - China and Rome - were also the least experienced in terms of previous trade interactions and cosmopolitan disease. To use the earlier-noted Pima and Papago Indian terminology, both China and Rome were well adapted to their own "staying sicknesses," but unlike the more trade-experienced peoples in much of India and the Middle East they had little experience with the "wandering sicknesses" that existed outside their realms. As a consequence, whereas most of India and the Middle East seem to have experienced no major pathogenic demographic reactions to the new expansion of commerce, China and Rome were convulsed by it.

In the late second century A.D. both Rome and China were probably overwhelmed by pestilence. In Rome the so-called Antonine plagues of A.D. 165-80 were followed less than a century later by another round of empirewide pandemic. Although it is impossible, at this distance, to determine what diseases entered Rome and caused such havoc during these times, opinion has long centered on smallpox or something similar or ancestral to it as the primary agent (Hirsch 1883; McNeill 1976). No doubt, however, other newly introduced infections (including, probably, measles) were present as well. The result was severe and prolonged population decline and, perhaps consequently, political upheaval.

A very similar epidemiological and social pattern developed in China during the same early centuries of this era (McNeill 1976). In short, the mobility of commerce, to a degree unprecedented in previous world history, introduced new and devastating waves of microparasitism to geographic locales that previously had existed in widely separated realms; the consequent population collapses probably gave rise to upheaval in the macroparasitic equilibrium (however uneasy it may already have been), with eventual deterioration and collapse of the existing political orders - the Roman and Han empires - altogether.

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