Origins of the disease
The plague disease, caused by
Yersinia pestis, is
enzootic (commonly present) in populations of fleas carried by ground
marmots, in various areas including
Northern India and
Uganda. Due to climate change in Asia, rodents began to flee the dried out grasslands to more populated areas, spreading the disease.
Nestorian graves dating to 1338–1339 near
Lake 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.
 In October 2010, medical geneticists suggested that all three of the great waves of the plague originated in China.
 In China, the 13th-century
Mongol conquest caused a decline in farming and trading. However, economic recovery had been observed at the beginning of the 14th century. In the 1330s, a large number of natural disasters and plagues led to widespread famine, starting in 1331, with a deadly plague arriving soon after.
 Epidemics that may have included plague killed an estimated 25 million Chinese and other Asians during the 15 years before it reached
Constantinople in 1347.
The disease may have travelled along the
Silk Road with
Mongol armies and traders or it could have come via ship.
 By the end of 1346, reports of plague had reached the seaports of Europe: "India was depopulated,
Armenia were covered with dead bodies".
Plague was reportedly first introduced to Europe via
Genoese traders at the port city of
Kaffa in the
Crimea in 1347. After a protracted siege, during which the Mongol army under
Jani Beg was suffering from the disease, the army catapulted the 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.
 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.
Geoffrey the Baker, Chronicon Angliae
The seventh year after it began, it came to England and first began in the towns and ports joining on the seacoasts, in Dorsetshire, where, as in other counties, it made the country quite void of inhabitants so that there were almost none left alive.
... But at length it came to Gloucester, yea even to Oxford and to London, and finally it spread over all England and so wasted the people that scarce the tenth person of any sort was left alive.
There appear to have been several introductions into Europe. The plague reached Sicily in October 1347, carried by twelve Genoese galleys,
 and rapidly spread all over the island. Galleys from
Kaffa reached Genoa and Venice in January 1348, but it was the outbreak in
Pisa a few weeks later that was the entry point to northern Italy. Towards the end of January, one of the galleys expelled from Italy arrived in
From Italy, the disease spread northwest across Europe, striking France, Spain, Portugal and England by June 1348, then turned and spread east through Germany and Scandinavia from 1348 to 1350. It was introduced in Norway in 1349 when a ship landed at
Askøy, then spread to Bjørgvin (modern
 Finally it spread to northwestern Russia in 1351. The plague was somewhat less common in parts of Europe that had smaller trade relations with their neighbours, including the majority of the
Basque Country, isolated parts of Belgium and the Netherlands, and isolated alpine villages throughout the continent.
Modern researchers do not think that the plague ever became endemic in Europe or its rat population. The disease repeatedly wiped out the rodent carriers so that the fleas died out until a new outbreak from Central Asia repeated the process. The outbreaks have been shown to occur roughly 15 years after a warmer and wetter period in areas where plague is endemic in other species such as
Middle Eastern outbreak
The plague struck various regions in the Middle East during the
pandemic, leading to serious depopulation and permanent change in both economic and social structures. As it spread to western Europe, the disease entered the region from southern Russia also. By autumn 1347, the plague reached
Alexandria in Egypt, probably through the port's trade with
Constantinople, and ports on the
Black Sea. During 1347, the disease travelled eastward to
Gaza, and north along the eastern coast to cities in
Aleppo. In 1348–1349, the disease reached
Antioch. The city's residents fled to the north, most of them dying during the journey, but the infection had been spread to the people of
Mecca became infected in 1349. During the same year, records show the city of
Mawsil (Mosul) suffered a massive epidemic, and the city of
Baghdad experienced a second round of the disease. In 1351
Yemen experienced an outbreak of the plague, coinciding with the return of
Sultan al-Mujahid Ali of Yemen from imprisonment in
Cairo. His party may have brought the disease with them from Egypt.