<Tr> <Td> </Td> <Td> The lead section of this article may need to be rewritten . The reason given is: it does not conform to WP: LEAD: it is not a summary of the article and is too long . Please discuss this issue on the article's talk page . Use the lead layout guide to ensure the section follows Wikipedia's norms and to be inclusive of all essential details . (November 2015) (Learn how and when to remove this template message) </Td> </Tr> <P> The Black Death, caused by Yersinia pestis, is enzootic (commonly present) in populations of ground rodents in Central Asia . Morelli et al. (2010) reported the origin of the plague bacillus to be in China . An older theory places the first cases in the steppes of Central Asia, and others, such as the historian Michael W. Dols, argue that the historical evidence concerning epidemics in the Mediterranean and specifically the Plague of Justinian point to a probability that the Black Death originated in Central Asia (not Africa - Asia (http://www.medicalnewstoday.com/articles/206309.php) where it then became entrenched among the rodent population . </P> <P> Nevertheless, from Central Asia it was carried east and west along the Silk Road, by Mongol armies and traders making use of the opportunities of free passage within the Mongol Empire offered by the Pax Mongolica . It 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, bringing the plague by ship into Sicily and the south of Europe, whence it spread . </P> <P> Regardless of its origin, it is clear that several preexisting conditions such as war, famine, and weather contributed to the severity of the Black Death . In China, the 13th century Mongol conquest disrupted farming and trading, and led to widespread famine . The population dropped from approximately 120 to 60 million . The 14th century plague is estimated to have killed 1 / 3 of the population of China . </P>

Over what land route did the bubonic plague spread from asia to europe