Plagues and Peoples (17 page)

Nevertheless, in that single season the disease inflicted a blow on Athenian society from which it never entirely recovered. This unforeseen and unforeseeable epidemiological accident, as Thucydides implies, may have had much to do with the failure of Athenian plans for the defeat of Sparta and the Peloponnesian League. Had Athens won that war, how different the subsequent political history of the Mediterranean would have been! But as things turned out, the Athenian empire, which lasted no more than three generations, was far more evanescent on a human time scale than the epidemic of 430–429
B.C
. was on a time scale appropriate to the life span of infectious disease-causing organisms. When the mysterious epidemic had come and gone, it left no trace behind, and for a long time the Mediterranean did not witness anything comparable.

The epidemic experience of China cannot be reconstructed with so much detail, although mention of unusual outbreaks of disease abound in the Han dynastic history and other early texts. Phrases used to describe such episodes cannot be translated into modern medical terminology. All that can be concluded is that China too, like the Mediterranean, had a substantial acquaintance with disease in different forms, including some that acted in epidemic fashion from time to time.
³⁷

Texts from ancient India tell nothing about the antiquity of epidemic infection in that land. Surviving medical writings claim immemorial antiquity, but were subject to a lengthy process of amendment and interpolation via oral transmission.
³⁸
Hence passages that have sometimes been cited as evidence for an enormous antiquity for smallpox and similar diseases in India do not prove any such thing. To be sure, on a
priori
grounds it is easy to believe that India offered particularly
fertile soil for the development of civilized person to person diseases. Warm climatic conditions, such as prevail in India, are obviously more propitious for the survival of tiny infectious organisms (attuned to flourish at body temperature) in their moment of peril, when transfer from one host to another occurs. Hence it was undoubtedly easier in India than in colder climates for infections established among cattle and other herds to transfer to human hosts. In no other warm climate were there dense enough human settlements living in a close juxtaposition with suitable animal herds in those centuries when a disease such as smallpox must have first made good its transfer to humankind. The modern tradition that smallpox was indigenous to India may therefore rest on a perfectly sound basis in fact.
³⁹
Bubonic plague and cholera, as we shall see, perhaps also began their careers as human diseases on Indian soil. But the universal human penchant for attributing the origin of an unfamiliar, nasty disease to foreigners makes it impossible to track the origins of any particular infection to India (or anywhere else) with any sort of historically convincing textual evidence.
⁴⁰
,
⁴¹

As for the Middle East, the biblical passages already cited show that epidemics were quite familiar there during the first millennium
B.C
. Clearly, the same infection could, on occasion, devastate both Middle Eastern and Mediterranean lands, as Thucydides declares to have been the case with the disease that attacked Athens in 430
B.C
. Perhaps an epidemic might occasionally also leap across the thinly populated ground separating India from the Middle East and the Mediterranean. It is even conceivable that China, too, may sometimes have shared exposure to a contagious infection.
⁴²
In general, however, any such sporadic bridging of epidemiological barriers separating the major centers of Eurasian population remained exceptional before the Christian era.

To be sure, within such well-traveled waters as the Mediterranean, movement by sea could, with favoring winds, attain an average of well over 100 miles per day.
⁴³
Thus, all the coastal cities of the Mediterranean constituted a single disease
pool. A person seemingly in good health at the time of embarkation might fall sick en route and communicate his illness to others on board. Shipboard travel could therefore easily carry an infection from one port to another, across hundreds or thousands of miles of water.

Overland travel, on the other hand, was slower, and persons falling ill could be left behind more easily en route. For both these reasons, disease traveled less easily overland than by sea. Nevertheless, long distance human travel, whether by land or sea, implied the possibility of bringing new infections to hitherto virgin host populations. Before the Christian era, however, regular movement between India, China, and western Eurasia did not attain any sort of stable organization. Opportunities for spreading infectious diseases from one part of the civilized world to another therefore remained exceptional and sporadic.

Under all ordinary circumstances thinly populated areas effectively insulated one center of dense human concentration in ancient Eurasia from others, since civilized person-to-person infections could not maintain themselves for any length of time amid a thinly scattered human host population. Indeed, even within what we are accustomed to think of as a single civilization, it is entirely probable that infections permanently residing in a large city or group of cities failed to establish themselves elsewhere on an endemic basis, but only invaded the less densely populated provinces sporadically, when a susceptible age group had become numerous enough to constitute a suitable field for infection.

We must therefore imagine that within each civilized region an ever-shifting microparasitic balance prevailed. Disease incidence must have altered as antibodies appeared and disappeared from human bloodstreams in response to individual encounters with infection. Simultaneously, genetic selection among both parasites and hosts operated to change disease behavior, and such factors as climate, human diet, human density, and patterns of movement all must have also im-
pinged upon the sensitive and unstable equilibrium between disease organisms and their human hosts.

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 immunity. All that was needed to provoke spillover from one pool to another was some accident of communication permitting a chain of infection to extend to new ground where populations were also sufficiently dense to sustain the infection either permanently, or at least for a season or two. The plague of Athens seemed to have been such an episode; others undoubtedly occurred in India, China, and elsewhere without leaving any trace we can discover today.

When, however, travel across the breadth of the Old World from China and India to the Mediterranean became regularly organized on a routine basis, so that thousands of individuals began to make a living by traveling to and fro, both on shipboard and by caravan, then conditions for the diffusion of infections among the separate civilizations of the Old World altered profoundly. The possibility of homogenization of those infections, whose most critical limit was defined by the number of new human hosts available day in and day out, opened up. It is my contention that something approximating this condition did in fact occur, beginning in the first century a.d.

Unfortunately, most details of how communications among the separate civilizations of the Old World developed between 200
B.C
. and
A.D
. 200 remain obscure. Only a few surprising events were recorded. We know, for instance, of a Chinese explorer who in 128
B.C
. reached the fertile valley of Ferghana in what is now Afghanistan. He was followed by an army detachment to garrison that distant outpost of Chinese imperial power, beginning in 101
B.C
. But a detachment of soldiers, each of whom had in all probability long since recovered from locally prevalent childhood diseases, were not very likely to be able to transfer an unfamiliar infection across the
thousands of miles between China and the Middle East. For such an event to become probable, a far more variegated bridge of human travelers was needed so as to scatter susceptibles all across the intervening distances in sufficient number to permit an infectious chain to run all across Asia.

Only when caravan trade became well established could such conditions arise. Nearly two centuries elapsed before regular and relatively large-scale trade came to be organized between China and Syria, following the routes these Chinese imperial emissaries had traversed. Costs of such travel were large. Camels and caravan personnel had to be maintained throughout the months of plodding between northwestern China and western Asia. Protection from confiscation en route had to be arranged. This meant payments for protection that were large enough to maintain formidable bodies of professional military men along the way. Last but not least, large numbers of persons had to have adequate motivation for undertaking such arduous enterprise: profit, adventure, imperial command or some combination of these inducements had to exert a stable impulse upon suitable numbers of men before regular exploitation of the possibility of moving back and forth between eastern and western Asian centers of civilization became a reality. Of these, profit was the most pervasive and, for long-continued enterprise, probably the most dependable. Profitable trading in turn depended on the supply and demand for goods valued highly enough in each civilized community that they could command the prices needed to meet the risks and costs of such a long and dangerous journey.

There is some evidence in Chinese texts to suggest that the opening to the West was exploited from the Chinese side with some vigor for a brief time after 126
B.C.
, but soon broke down, when the impulse of imperial command slackened. Then during the first century
A.D.
, movement picked up again. New and more stable political conditions established themselves throughout the length of what Romans soon began to call the Silk Road, since silk from China became the principal commodity carried westward in this manner. This trade
reached a climax about
A.D
. 100, as the ladies of Rome and other Mediterranean cities began to dress themselves in semitransparent silks. These were produced in Antioch by unraveling stout silk cloth imported from China and reweaving the thread into a loose web that achieved the desired transparency.
⁴⁴

The establishment of a regular caravan trade across Asia had important consequences for the continent’s macroparasitic patterns. Traders accompanying their goods could be taxed, and taxed they were by local potentates along the way. Protection payments (whether in kind or in cash) hired guards; when such guards were not actually engaged in accompanying caravans, they were of course available for enforcing and extending their leader’s sway at the expense of rivals. Trade thus sustained and provoked political consolidation of a string of states extending along the caravan route, all the way from Roman Syria to the northwestern border of China.

Successful rulers within this belt of semi-desert lands were either steppe nomads themselves or but recently descended from such folk. (Nomadry encouraged, indeed required courage and other military virtues for defense of herds and pasturage, and their horses gave nomads a mobility superior to that which cultivators could attain, making concentration of superior force in course of a sudden raid relatively easy.) Interpenetration between nomad tribesmen of the steppelands and masters of the oases of central Asia became correspondingly intimate; state structures of hitherto unequaled extent and stability resulted.
⁴⁵

For a long time the resulting symbiosis was delicate and liable to frequent upset. By taking too much from caravan personnel, merchants’ incentive to undertake the risks of travel could be snuffed out. Yet by not paying enough to support a superior military establishment along the trade route, merchants invited more distant nomad groups to push southward from the open steppe and try to seize as booty what they were not yet in a position to tax as rulers. The instability was not unlike the ecological instability characteristic
of a new infection. And as is also the case with many new infections, fully stable trade and protection systems were never achieved. It is not therefore really surprising that the pace of trade seems to have slackened even before the middle of the second century
A.D
., owing to political (and perhaps epidemiological) difficulties along the way.
⁴⁶

Organization of sea contacts between Mediterranean, Indian, and Chinese peoples proceeded on almost the same temporal rhythm. A Greek explorer “discovered” the monsoons of the Indian Ocean some time before the Christian era. Thereafter, traders whom the Indians called “Yavanas,” that is, “Ionians,” continued to appear along Indian coasts, issuing from ports on the Red Sea, though it is impossible to estimate the number and frequency of such voyages. Other seafarers opened sea communications across the Bay of Bengal and throughout the South China Sea. Peoples of Indonesia and the southeastern Asian mainland took a leading part in this development, although seafarers living in India itself also participated.

One conspicuous result of the development of seafaring in the Indian Ocean and the South China Sea was the transplantation of Indian court culture to the river valleys and some of the islands of southeastern Asia, beginning not long before the Christian era. Broad new regions, climatically warmer and sometimes wetter, but otherwise quite similar to the Ganges Valley, thus opened up for civilized development. For many centuries the new states of southeastern Asia remained relatively isolated transplants, surrounded by untamed jungle whose slow retreat before agricultural settlement is still incomplete in our own time. The comparative slowness of civilized expansion in this environment is almost certainly connected with the health consequences of trying to concentrate dense human populations within a well-watered tropical landscape. Intensification of microparasitism—with malaria and dengue fever perhaps in the lead, water-borne infections of the alimentary tract close behind, and an extremely complex series of multicelled parasites available to batten upon what
remained—presented formidable obstacles to the growth of population in southeastern Asia toward anything like the densities that sustained Chinese and Indian civilizations. Or so one may legitimately infer from the fact that strong and massive states equivalent to the Chinese or even Indian empires did not in fact arise in southeastern Asian river valleys at any time, despite the obvious fact that the geographical areas in question provided ample space for a powerful civilization to arise there.
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