Origins of the disease
Main article: Black Death migration
The plague disease, caused by Yersinia pestis, is enzootic (commonly present) in populations of fleas carried by ground rodents, including marmots, in various areas, including Central Asia, Kurdistan, Western Asia, North India, and Uganda.[10] Due to climate change in Asia, rodents began to flee the dried-out grasslands to more populated areas, spreading the disease.[11] Nestorian graves dating to 1338–1339 near Issyk-Kul in Kyrgyzstan have inscriptions referring to plague and are thought by many epidemiologists to mark the outbreak of the epidemic, from which it could easily have spread to China and India.[citation needed] In October 2010, medical geneticists suggested that all three of the great waves of the plague originated in China.[12]

The 13th-century Mongol conquest of China caused a decline in farming and trading. Economic recovery had been observed at the beginning of the fourteenth century. In the 1330s, many natural disasters and plagues led to widespread famine, starting in 1331, with a deadly plague arriving soon after.[13] Epidemics that may have included the plague killed an estimated 25 million Chinese and other Asians during the fifteen years before it reached Constantinople in 1347.[14][15]

The disease may have travelled along the Silk Road with Mongol armies and traders or it could have arrived via ship.[16] By the end of 1346, reports of plague had reached the seaports of Europe: “India was depopulated, Tartary, Mesopotamia, Syria, Armenia were covered with dead bodies”.[17]

Plague was reportedly first introduced to Europe via Genoese traders from the port city of Kaffa in the Crimea in 1347.[18][19] During a protracted siege of the city by the Mongol army under Jani Beg, whose army was suffering from the disease, the army catapulted infected corpses over the city walls of Kaffa to infect the inhabitants. The Genoese traders fled, taking the plague by ship into Sicily and the south of Europe, whence it spread north.[20] Whether or not this hypothesis is accurate, it is clear that several existing conditions such as war, famine, and weather contributed to the severity of the Black Death.