Between 1347 and 1353 the Black Death, one of the deadliest pandemics in human history, killed thirty to sixty percent of Europe's population. For centuries the epidemic continued to strike every 10 years or so, its last major outbreak being the Great Plague of London from 1665 to 1666. Though the vectors were not understood at the time, the disease was spread by rats and transmitted to people by fleas or, in some cases, directly by breathing.
"The pandemic is thought to have begun in Central Asia, and spread to Europe during the 1340s. The total number of deaths worldwide is estimated at 75 million people, approximately 25–50 million of which occurred in Europe. . . . It may have reduced the world's population from an estimated 450 million to between 350 and 375 million in 1400.
"The 14th century eruption of the Black Death had a drastic effect on Europe's population, irrevocably changing the social structure. It was a serious blow to the Roman Catholic Church, and resulted in widespread persecution of minorities such as Jews, foreigners, beggars, and lepers. The uncertainty of daily survival created a general mood of morbidity, influencing people to 'live for the moment', as illustrated by Giovanni Boccaccio in The Decameron (1353)" (Wikipedia article on Black Death, accessed 01-03-2009).
"The three plague waves [Plague of Justinian, Black Death, and that beginning in China's Yunnan province in 1894] have now been tied together in common family tree by a team of medical geneticists led by Mark Achtman of University College Cork in Ireland. By looking at genetic variations in living strains of Yersinia pestis, Dr. Achtman’s team has reconstructed a family tree of the bacterium. By counting the number of genetic changes, which clock up at a generally steady rate, they have dated the branch points of the tree, which enables the major branches to be correlated with historical events.
"In the issue of Nature Genetics published online Sunday [October 31, 2010], they conclude that all three of the great waves of plague originated from China, where the root of their tree is situated. Plague would have reached Europe across the Silk Road, they say. An epidemic of plague that reached East Africa was probably spread by the voyages of the Chinese admiral Zheng He who led a fleet of 300 ships to Africa in 1409 (http://www.nytimes.com/2010/11/01/health/01plague.html, accessed 11-01-2010).