What does this mean, 'jumped to humans'?
I've just been reading Ole Benedictow's book, by the way, I'd recommend it.
Modern epidemics of rodent plague/Yersinia pestis involve the rodent plague passing among rodent populations and occasionally accidentally affecting human populations. Unusually these tend to have higher mortality in smaller communities than in towns and cities, because the rodent population and its density determines the course of the rodent plague. Some modern epidemics involve the rodent plague passing among human populations, but much less effectively than among rodent populations.
One of the reasons the rodent plague/Yersinia pestis has been identified as the plague, besides the symptoms, and besides the presence of Yersinia pestis in some plague cemetaries, is because the Black Death also had higher mortality in smaller communities than in towns and cities, and because the Black Death caused far greater relative mortality in its first outbreak in each region than in the following outbreaks; this is strong evidence that it's animal hosts' population and its density determined the course of the initial outbreak, and having been hit even harder than the human population, limited the severity of the following outbreaks. [One of the counterarguments is that the Black Death seems to have conferred relative immunity to its survivors which might explain the pattern in the first generation but not the rebounding population in subsequent generations.]