The Black Death was one of the most devastating pandemics in human history, resulting in the deaths of an estimated 75 to 200 million people and peaking in Europe in the years 1346–53.Although there were several competing theories as to the etiology of the Black Death, analysis of DNA from victims in northern and southern Europe published in 2010 and 2011 indicates that the pathogen responsible was the Yersinia pestis bacterium, probably causing several forms of plague. The Black Death is thought to have originated in the arid plains of Central Asia, where it then travelled along the Silk Road, reaching Crimea by 1343. From there, it was most likely carried by Oriental rat fleas living on the black rats that were regular passengers on merchant ships. Spreading throughout the Mediterranean and Europe, the Black Death is estimated to have killed 30–60% of Europe's total population.In total, the plague reduced the world population from an estimated 450 million down to 350–375 million in the 14th century. The world population as a whole did not recover to pre-plague levels until the 17th century. The plague recurred occasionally in Europe until the 19th century. The plague created a series of religious, social, and economic upheavals, which had profound effects on the course of European history 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, Northern India and Uganda.Nestorian graves dating to 1338–9 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 which 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, Tartary, Mesopotamia, Syria, 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. 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 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) and Iceland.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 Kingdom of Poland, 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 gerbils.

• published: 09 Nov 2015
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