Maggie's Farm

the end of the medieval warm period, combined with several seasons of heavy rains happened at a time when populations in Europe had expanded to the limit of the carrying capacity of agriculture. The rains caused repeated crop failure in the early part of the 14th century, resulting in wide spread famine, including the Great Famine of 1315–17.

the population that lived through the famines was not at peak, healthwise, when plague broke into the Med. aggravating that fact, Plague had disappeared from Europe since about the 8th century, so immunological responses in the population (exposure and survival confer some immunity) were gone, as was practical experience in dealing with that kind of infection.

epidemic disease tends to be most lethal when it appears in populations without prior exposure, and plague is no different.

plague is better understood as a relatively stable cycle of infection among rodents, fleas and y. pestis. plague appears in the human communities when something disturbs this stable cycle, such as war, displaced populations who do not have experience in living alongside plague reservoirs (for example, a local hunter would have learned not to trap or keep sick animals), etc. stable meaning, infected hosts do not die off quickly wholesale, and that is the situation in rodent populations in the asian steppes and the American west, which are old and new plague reservoirs. because humans aren't part of this normal cycle, there are no evolutionary pressures to moderate lethality, and people get collected in an epidemic like a NASCAR mass wreck.

the strains of plague that caused the Plague of Justinian and the Black Death are extinct.