Introduction of New Diseases

The introduction of new diseases to a virgin population can be calamitous. The most celebrated case is known as the "great dying" that followed the introduction of Old World diseases to the New World in the sixteenth century. But contact between virulent disease-causing organisms and nonimmune populations must have been repeated over and over again in the course of human history as civilizations with different disease ecologies came into contact with one another. Thus it is appropriate to ask whether East Asian populations also suffered major demographic catastrophes as a result of the introduction of unfamiliar disease organisms.

Undoubtedly there was such a time, but Chinese historians have not defined it as such. A likely time for such an occurrence was around the beginning of the Christian era, when growth and expansion of the early empire brought China into more frequent contact with neighboring peoples to the south. By the first century B.C., a string of populous communities stretched from South China to Bengal, and by the first century A.D., traders and Buddhist missionaries were traveling overland from northern India to China through Central Asia and by way of Vietnam. Chinese scholars also traveled to India to study at Buddhist monasteries, and they returned with foreign diseases as well as foreign philosophies.

One of these foreign diseases, possibly the first to be reported in Chinese sources, was smallpox. Smallpox, a prominent, density-dependent disease of the past, is used here as a paradigm for disease in general, because it was highly visible and because changes in its incidence and distribution reflect changing disease ecologies.

It is not clear exactly when smallpox first reached China, but at least two early references to it are known. Donald Hopkins (1983) suggests that a disease called "Hun pox" that came to China from the north around 250 B.C. may have been smallpox. He also suggests a second introduction of smallpox from the south about A.D. 48, when a Chinese general and half his troops lost their lives to a "barbarian" disease as they were putting down a rebellion in southwest China. The first unmistakable description of smallpox comes from an account written in the fourth century A.D. (Leung, in this volume, VI.2). However, given the extent of earlier Chinese contact with India - where it is believed this disease was known before 400 B.C. - smallpox must have reached China earlier than the fourth century.

We do not need to know precisely when smallpox first reached China to understand that early smallpox epidemics would have been very costly in terms of human life. Smallpox normally produced a 25 percent case-fatality rate, and in virgin-soil epidemics the rate would have been much higher. Thus given China's population size and the links between many centers of density, we can be certain that the introduction of smallpox to China caused a demographic crisis of considerable magnitude.

Population trends for the early empire suggest that there were several demographic crises that might have been caused by unfamiliar diseases. Periods of population growth alternated with periods of population decline: From the Warring States period through the Tang dynasty, China's population fluctuated, reaching and then retreating from a maximum of about 60 million people. The periods of decline during these centuries have been associated with natural disasters and China's periodic wars, but high mortality from disease, undoubtedly exacerbated by war and famine, certainly contributed to population decline.

After the initial shock waves subsided, however, the demographic characteristics of China's population would have ensured that smallpox became an endemic disease rather quickly. Hopkins estimates that a population of less than 300,000 people was sufficient to maintain smallpox as an endemic disease, because enough new susceptibles would have been borne each year to sustain the chain of smallpox infection indefinitely. As an endemic disease, smallpox would have circulated throughout the empire, appearing most frequently in the larger cities and least often in the sparse populations of the hinterlands. We know that smallpox continued to claim numerous lives in China for many centuries; however, once smallpox and other infectious diseases became endemic, the risk of a catastrophic demographic crisis would have been sharply reduced.

The history of smallpox in East Asia reveals a curious chronology in the transmission of this disease. Of particular interest is the fact that smallpox is documented much earlier in the ancient civilizations of the West. Egyptian mummies 3,000 years old have been found with scars that resemble the typical pockmarks left by smallpox; and the people of ancient Greece, as well as the people of India, are believed to have been afflicted with this disease before 400 B.C. The much later documentation of smallpox in China suggests that smallpox spread to eastern Asia from the West - a journey, if estimates of when smallpox reached China are even close to being correct, that took more than a thousand years.

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