" Unlike viral infections, which often left behind a large core of immune survivors to care for the ill and harvest the food the next time an epidemic struck, plague spared no one. Despite the findings about CCR5-D32, the best available current evidence is that Y. pestis does not produce permanent immunity in victims. During the Black Death, this biological quirk may have produced an enormous secondary mortality. As both Boccaccio and Stefani suggest, many people seem to have died not because they had particularly virulent cases of plague, but because the individuals who normally cared for them were either dead or ill themselves. "

― , The Great Mortality: An Intimate History of the Black Death, the Most Devastating Plague of All Time