Although the plague spreads to human populations from fleas that infest black rats, the plague bacillus, Pasteurella pestis, is fatal to humans and rats and hence needs a separate long-term reservoir. In nature it exists as an endemic disease in burrowing rodent populations. In the 20th century, for example, after spreading by ship from Hong Kong to port cities of North and South America, it became nativized among Andean and Rocky Mountain ground squirrels and marmots. Since plague outbreaks occasionally reached the Mediterranean but never became a constant threat before the great outbreak of 1347, the plague bacillus, now endemic among marmots in the neighboring Black Sea steppe zone, probably became nativized there only in the 14th century. From then on the burrowing rodents of the Black Sea and Caspian steppes served as reservoirs for constant outbreaks in western Eurasia until trade and lifestyle changes occurred in the 17th century.
The 14th-century Black Death first appeared in Mongol-ruled China. From 1313 a series of epidemics struck Henan province; they culminated in 1331 with an epidemic that supposedly killed nine-tenths of the population. Epidemics broke out in coastal provinces in 1345–46. Finally, in 1351 massive epidemics began to strike throughout China yearly up to 1362, causing catastrophic population decline. William McNeill has thus speculated that the plague was originally native to burrowing rodents of the Himalayan foothills. The Mongols, by joining YUNNAN on the southeastern skirts of the Himalayas to China proper and hunting marmots there, inadvertently transmitted the plague to Henan and the Chinese heartland by 1331, if not before. From there Mongol activity introduced it into the marmot colonies of Inner Asia, whence it began to spread west. European and Muslim writers virtually all recorded the plague as beginning in China and then crossing the steppe to the Crimea. Excavations of a Christian cemetery near Ysyk-Köl Lake (Kyrgyzstan) suggest a devastating outbreak of plague in 1338–39. Muslim writers noted the progress of the plague from KHORAZM in 1345 to the center of the Golden Horde in 1346 and south to Mongol soldiers in Azerbaijan in 1346–47. Mongol military operations then spread it to Mosul and Baghdad in 1349. Early outbreaks in Sindh had probably followed caravan routes south from Khorazm; evidence of an Indian Ocean transmission route is slim.