Although there were several competing theories as to the etiology of the Black Death, recent analysis of DNA from victims in northern and southern Europe indicates that the pathogen responsible was the Yersinia pestis bacterium, which causes the Bubonic plague, although these were different, previously unknown ancestral variants of those identified in the 20th century.
The Black Death is thought to have started in China or central Asia, before spreading west. The plague then travelled along the Silk Road and reached the Crimea by 1346. From there, it was probably 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 percent of Europe's population. All in all, the plague reduced the world population from an estimated 450 million to between 350 and 375 million in the 14th century.