From the New York Times:
“The Black Death began in Europe in 1347 and carried off an estimated 30 percent or more of the population of Europe. 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. The disease is spread by rats and transmitted to people by fleas, or, in some cases, directly by breathing.
In the issue of Nature Genetics published online Sunday, [the researchers] 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.”
For all the details, check out this article.