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

by John Kelly

Most of the time Y. pestis and the rodent kingdom live in a state of unhappy but workable coexistence. The scientific term for the modus vivendi is enzootic. Animals continue to get sick and die, but usually there are enough partially resistant rodents in any given community to keep the smoldering embers of infection in check. There are a number of theories about why, from time to time, this biological firewall suddenly collapses and the colony is consumed by the flames of an epizootic—a full-scale outbreak of plague. These include genetic change in the plague bacillus, which makes it more virulent, and demographic changes in the wild rodent community, which make its members more vulnerable to plague. A third theory, not incompatible with the first two, is that epizootics are activated by surge years—sudden, dramatic spurts in the rodent population. No one is sure what causes surges, but a number of scientists believe that they may be related to sun spots, which follow roughly the same cyclical pattern as surge years in many (though not all) rodent species, approximately ten to twelve years.