"In the area south of the Yangtse, the land is low and the climate humid; adult males die young"
-- Ssu-ma Ch'ien, father of Ch. historiography (145-87 B.C.):
By 500 B.C.E., the earliest civilizations on the Middle East had expanded their empires to the limits of the Middle East.and a fairly stable balance had been achieved where they had at least partially adapted to the parasitic and infectious diseases of the region and evolved into "childhood diseases" where concentrations of populations allowed them to survive as a chronic, but usually not fatal, nuisance.
However, the newer areas of Eurasia where civilization spread, the Yellow River in China and the Mediterranean in the West, were relatively free of infectious diseases. This was largely because wheat, and barley, the main crops grown there, were native to the regions, thus causing relatively little biological disruption. This contrasted with areas that practiced irrigation for non-native crops, which exposed people to water borne parasitic diseases. Although there was irrigation in the Yellow River area of China, its cooler and drier climate led to considerably fewer problems with parasitic diseases than the earliest hydraulic civilizations encountered. However, when the Chinese spread southward into the Yangtze River region with its hotter and more humid climate, they encountered water and insect borne diseases to which it took centuries for them to adapt.
In the second century C.E , as Rome and China established trade links across Eurasia, they also encountered the older infectious diseases of the older civilizations in between. As a result, these diseases spread to the eastern and western fringes of Eurasia with very similar results.
In the East, small pox and measles, diseases never previously encountered there, hit China in 161, 310, and 322. Such unprecedented disasters led people to question traditional Chinese beliefs and opened the way for the rise of Buddhism. The severe population loss these outbreaks caused also contributed to the fall of the Han Dynasty and several centuries of turmoil, until the revival under the Sui and T’ang Dynasties. However, in the 600s, the eruption of a new disease, arising from India or Africa, bubonic plague, caused another huge loss of life and probably contributed to the decline and fall of the T’ang Dynasty by 906. After 900, the Chinese had adapted somewhat to this scourge, and China saw its population and towns expand rapidly under the Sung Dynasty.
In the West, the Roman Empire also suffered the initial onset of smallpox and measles in 165 and 251, helping lead to a period of anarchy in the third century and the eventual fall of the Empire by 500. And, like in China, people questioned the old pagan religions, leading the growing popularity and eventual triumph of Christianity. Then, as population and prosperity were recovering, especially in the Eastern Mediterranean, bubonic plague hit in the 500s, allowing the sudden rise of the Arab Muslims in the 600s. By 900, the plague had also subsided in the Mediterranean and Western Europe, allowing the revival of towns and trade.
Unfortunately, these newly revived cities with their concentrations of populations were especially vulnerable to onset of a new strain of the plague in the 1300s, triggering possibly the greatest demographic disaster in history.