Disease Ecologies of East Asia

East Asian scholars have begun only recently to examine Chinese, Korean, and Japanese sources for evidence of the history of disease in East Asia. Research is at a very early stage: There is much that we do not know, and some of what we think we know may turn out to be wrong. At present, scholars disagree about basic facts as well as about how to interpret them. It is possible, however, to discuss how disease ecologies changed as East Asian civilization developed, and this essay will consider how long-term historical change in East Asia altered the disease ecologies of this major world region.

East Asia is a large ecological niche bounded on all sides by less hospitable terrain. To the north and northwest lie the vast steppe lands of Central Asia and the virtually impossible Takla Makan Desert. To the west lie the high Tibetan Plateau and the Himalayan Range with the world's highest mountains. To the south is the mountainous terrain of southwest China and the jungles of Southeast Asia. And to the east lies the Pacific Ocean. These formidable barriers and the great distances between eastern and western Eurasia long separated East Asia from the ancient civilizations of the West, and permitted a distinctive culture to develop and to spread throughout the region with relatively little influence from the outside.

East Asia can be divided into two major ecological zones. The northern zone encompasses the steppe and forest lands that lie north of China's Great Wall and today includes the modern regions of Inner Mongolia and Manchuria. This vast territory is unsuitable for agriculture, and it has long been the homeland of nomadic tribes who migrate with the seasons seeking grazing lands for their livestock. The steppe supports relatively few people, and thus has been sparsely settled throughout human history. Low population density and a pastoral, nomadic way of life have prevented the buildup of endemic foci of disease organisms in any one place.

Ecological conditions south of the Great Wall are altogether different. North China, which includes the areas drained by the Yellow River and its tributaries, as well as the northern tributaries of the Yangtze River, has a temperate, semiarid climate with hot summers and cold winters. Rainfall is lim ited and uncertain - about 400 to 800 millimeters per year. North China is most suitable for growing millet, wheat, and beans, and one or two crops a year can be grown in a growing season that lasts from 4 to 6 months.

South China, generally the region from the Yangtze Valley to the coast of southern China, has a subtropical climate, abundant rainfall (approximately 800 to 1,600 mm per year), and temperatures that are less extreme than those of North China. Rice is the dominant crop in the south, and two or three crops can be grown during 9 to 12 months of the year. Despite these differences in climate, North and South China have been bound together historically by a dominant, distinctive culture common to them both. And in historic times both regions have supported far greater population densities than the frontier regions north of China's Great Wall.

In prehistoric times Paleolithic humans roamed over all of East Asia. Early hunters and gatherers inhabited the forests and steppes of northeastern Asia, the highlands and plains of North China, the semitropical and tropical regions of South China, the Korean peninsula, and the Japanese islands. As long as primitive humans moved from place to place in search of food, the disease ecologies of East Asia must have been essentially unchanging. Although early humans may have been afflicted with maladies caused by microparasites indigenous to the different regions of East Asia, they did not suffer from many of the diseases with which we are familiar today, because the microparasites that cause them require large and concentrated host populations that did not yet exist.

The history of changing disease ecologies in East Asia begins with the emergence of Chinese civilization in late Neolithic times. In East Asia, as in other parts of the world, human society underwent a fundamental change in the late Neolithic period when the ancestors of the Chinese people began to engage in agriculture and to live in permanent communities. This transition from shifting to permanent places of residence occurred in at least two and possibly several regions of China about 6,000 years ago. The best known Neolithic sites are found in the loess region of North China near the juncture of the Wei and Yellow rivers; however, recent excavations in the Yangtze River delta have turned up evidence of Neolithic communities that may be even earlier than those of North China. The historic relationship between these cultures is not clear, but it was from these core regions that the culture we regard as distinctively East Asian evolved.

The Neolithic Revolution in China was clearly successful. Archeological evidence and early forms of Chinese ideographic script reveal an advanced society based on labor-intensive agriculture from the time of the Shang dynasty (1765-1122 B.C.). The need for labor led to dense concentrations of people and the need for protection to walled cities where grain could be stored and defended. These fundamental changes altered the size, density, and distribution of populations on the East Asian mainland. The growth of cities brought new problems: Human and animal wastes accumulated, drinking water became contaminated, parasites that thrive in stagnant water flourished, and people who now lived close together in permanent settlements became a reservoir for microparasites that cause human disease.

The range of Chinese expansion to the south and east was virtually unlimited. As indigenous peoples took up an agrarian way of life, more land was brought under cultivation and China's population grew. From the time of the Shang dynasty to the unification of China in 221 B.C., the Chinese people were ruled by great regional lords who controlled large territories in North and South China. There are no census statistics to serve as benchmarks, but other kinds of evidence indicate that even before unification China had produced a large population. The size of armies increased 10-fold during the Warring States period (421 to 403 B.C.), and by the fourth century B.C. each of the leading states of China was able to support a standing army of about 1 million men.

In 221 B.C., Shih Huang-t'i, ruler of the state of Ch'in, conquered all of China, unifying the states and regions of North and South China. He ordered the construction of large-scale public works designed to consolidate political control and to unify China economically. These projects, which included the construction of a national network of roads and the completion of the Great Wall on China's northern frontier, offer further evidence of an abundant labor force. The latter project alone required an estimated 700,000 laborers to build.

Long periods of warfare and large public works projects helped to homogenize the disease environment. The amassing of soldiers for battle and the concentration of many thousands of workers provided an ideal environment for disease organisms to propagate. And when the participants returned home, they carried diseases to the towns and villages through which they passed. Hence, the construction of roads and canals, whose function was to strengthen centralized control, promoted the spread of disease throughout the empire.

The Ch'in dynasty was short-lived, but the consolidation of political power under centralized imperial rule continued under the dynasties that followed. The four centuries of Han rule (202 B.C. to A.D. 220) were a period of expansion in which the Chinese way of life based upon labor-intensive, irrigated agriculture became the dominant culture on the mainland. And during the Sui (581-618) and T'ang (618-907) dynasties, the completion of the Grand Canal that connected the Yangtze and Yellow rivers reinforced the links between north and south China.

Urbanization was another prominent feature of social development in the early empire. By T'ang times, China had 21 cities with more than a half-million inhabitants. The largest urban concentration was at the T'ang capital of Ch'ang-an, which had 2 million registered inhabitants. Even smaller cities like the southern port of Canton and the bustling economic center at Yangchow, where the Grand Canal joined the Yangtze River, were centers of human density large enough to serve as reservoirs for many human disease pathogens.

In short, from very early times China had one of the world's largest populations and conditions of life that could support many of the density-dependent diseases of civilization with which we are familiar today. Indeed social, demographic, and environmental conditions in the early empire were such that China must have had a full range of indigenous diseases that varied by region according to the climate, the available host and vector populations, the density and distribution of human host populations, and the sanitary conditions in those regions. But for the most part, we do not know what diseases were present in the early empire.

We do know, however, that many different kinds of human disease existed in China from very early times, because they were written about on the Shang oracle bones that were used for divination (as indicated in Lu and Needham, in this volume, VI. 1). According to these ancient writings, the diseases prevalent at the time exhibited many different symptoms and were called by many different names. But whether they were caused by pathogenic agents that exist today, by related pathogens, or by unrelated pathogens is uncertain.

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