Populations in crisis

In Europe, the Medieval Warm Period ended some time towards the end of the 13th century, bringing the "Little Ice Age" and harsher winters with reduced harvests. In northern Europe, new technological innovations such as the heavy plough and the three-field system were not as effective in clearing new fields for harvest as they had been in the Mediterranean because the north had poor clay soil, and the potato, otherwise ideal for Northern Europe, was an American crop unknown in Europe at the time. Food shortages and rapidly inflating prices were a fact of life for as much as a century before the plague. Wheat, oats, hay and consequently livestock were all in short supply. Their scarcity resulted in malnutrition, which increases susceptibility to infections due to weakened immunity. Consistently high fertility rates, at five or more children per woman throughout Europe, resulted in high population growth rates and contributed to food shortages. In the autumn of 1314, heavy rains began to fall, followed by several years of cold and wet winters. The already weak harvests of the north suffered and the seven-year famine ensued. In the years 1315 to 1317, a catastrophic famine, known as the Great Famine, struck much of northwest Europe. It was arguably the worst in European history, reducing the population by perhaps more than 10 percent.

                                

Infection and migration

The plague disease, generally thought to be 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, Northern India and Uganda. Nestorian graves dating to 1338–9 near Lake Issyk Kul in Kyrgizstan 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 confirmed that the plague originated in Yunnan, province in southwest China. In China, the 13th century Mongol conquest caused a decline in farming and trading. However, economic recovery had been observed in the beginning of the 14th century. In the 1330s a high frequency of natural disasters and plagues led to widespread famine starting in 1331, with a deadly plague arriving soon after. The 14th-century plague killed an estimated 25 million Chinese and other Asians during the 15 years before it entered Constantinople in 1347. However, according to George Sussman, the first obvious medical description of plague in China dates to 1644.

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, Tartary, Mesopotamia, Syria, Armenia were covered with dead bodies".

Plague was reportedly first introduced to Europe at the trading city of Caffa in the Crimea in 1347. After a protracted siege, during which the Mongol army under Jani Beg was suffering the disease, they catapulted the infected corpses over the city walls 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.

European outbreak

There appear to have been several introductions into Europe. It reached Sicily in October 1347 carried by twelve Genoese galleys, where it rapidly spread all over the island. Galleys from Caffa 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 Marseille.

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 Bergen) but never reached Iceland. Finally it spread to northwestern Russia in 1351. The plague spared some parts of Europe, including the Kingdom of Poland and isolated parts of Belgium and the Netherlands.

Middle Eastern outbreak

The plague struck various countries 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 Lebanon, Syria and Palestine, including Ashkelon, Acre, Jerusalem, Sidon, Damascus, Homs, and Aleppo. In 1348–49, 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 Asia Minor.

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. This coincided with the return of King Mujahid of Yemen from imprisonment in Cairo. His party may have brought the disease with them from Egypt.