I believe the answer to this should actually be the Black Death. The bubonic plague is actually only part of the Black Death, which consisted of 3 'plagues' that all stemmed from the same bacteria carried by the fleas on the rats that arrived in Europe via the trading ships. Of those three, the bubonic plague was actually the least fatal. The pneumonic plague (very high mortality rate) and the septicaemic plague (almost always fatal) were also instrumental in the high death toll, so I don't think the answer can be just the bubonic plague. As sources, I am actually working from memory. I'm a teacher and taught this topic to my students last year, though I did do a quick search, and other sites support this view.