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Beyond goods and cultures, diseases too traveled the trade routes of Eurasia, and with devastating consequences. Each of the major population centers of the Afro-Eurasian world had developed characteristic disease patterns, mechanisms for dealing with them, and in some cases immunity to them. But when contact among previously isolated human communities occurred, people were exposed to unfamiliar diseases for which they had little immunity or few effective methods of coping. The epidemics that followed often brought suffering and death on an enormous scale to rich and poor alike. An early example involved the Greek city-state of Athens, which in 430–429 B.C.E. was suddenly afflicted by a new and still-unidentified infectious disease that had entered Greece via seaborne trade from Egypt, killing perhaps 25 percent of its army and permanently weakening the city-state.

Even more widespread diseases affected the Roman Empire and Han dynasty China as the Silk Roads promoted contact all across Eurasia. Smallpox and measles devastated the populations of both empires, contributing to their political collapse. Paradoxically, these disasters may well have strengthened the appeal of Christianity in Europe and Buddhism in China, for both of them offered compassion in the face of immense suffering.

Again in the period between 534 and 750 C.E., intermittent outbreaks of bubonic plague ravaged the coastal areas of the Mediterranean Sea as the black rats that carried the disease arrived via the seaborne trade with India, where they originally lived. What followed was catastrophic. Constantinople, the capital city of the Byzantine Empire, lost thousands of people per day during a forty-day period in 534 C.E., according to a contemporary historian. Disease played an important role in preventing Byzantium from reintegrating Italy into its version of a renewed Roman Empire encompassing the Mediterranean basin. The repeated recurrence of the disease over the next several centuries also weakened the ability of Christendom to resist Muslim armies from Arabia in the seventh century C.E.

The most well-known dissemination of disease was associated with the Mongol Empire, which briefly unified much of the Eurasian landmass during the thirteenth and fourteenth centuries C.E. (see Chapter 11). That era of intensified interaction facilitated the spread of the Black Death — identified variously with the bubonic plague, anthrax, or a package of epidemic diseases — from China to Europe. Its consequences were enormous. Between 1346 and 1348, up to half of the population of Europe perished from the plague. “A dead man,” wrote the Italian writer Boccaccio, “was then of no more account than a dead goat.”6 Despite the terrible human toll, some among the living benefited. Tenant farmers and urban workers, now in short supply, could demand higher wages or better terms. Some landowning nobles, on the other hand, were badly hurt as the price of their grains dropped and the demands of their dependents grew.

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A similar death toll afflicted China and parts of the Islamic world. The Central Asian steppes, home to many nomadic peoples, including the Mongols, also suffered terribly, undermining Mongol rule and permanently altering the balance between pastoral and agricultural peoples to the advantage of settled farmers. In these and many other ways, disease carried by long-distance trade shaped the lives of millions and altered their historical development. (See Chapter 11 for more on the Black Death.)

In the long run of world history, the exchange of diseases gave Europeans a certain advantage when they confronted the peoples of the Western Hemisphere after 1500. Exposure over time had provided them with some degree of immunity to Eurasian diseases. In the Americas, however, the absence of domesticated animals, the less intense interaction among major centers of population, and isolation from the Eastern Hemisphere ensured that native peoples had little defense against the diseases of Europe and Africa. Thus, when their societies were suddenly confronted by Europeans and Africans from across the Atlantic, they perished in appalling numbers. Such was the long-term outcome of the very different histories of the two hemispheres.