The Black Death was one of the most devastating pandemics in human history, peaking in Europe between 1348 and 1350. 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. It is estimated to have killed 25 million people or 30% of the population of China. 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.
Chapter 11 of what are you referring to.