Starting in 541–542 AD, the Plague of Justinian and a series of subsequent outbreaks killed between 13-26% of the world's population. The plagues were caused by Yersinia pestis, the same microorganism that would cause the Black Death in the 14th century. However, each subsequent outbreak during this period was less deadly than the previous; likely due to the propensity of pathogenic organisms to adapt in such a way that they actually don't kill their host!

My question is this: What mechanisms exist that might have made a subsequent outbreak of Yersinia pestis more virulent and more deadly that those that preceded it?