Plague has often been used as a synonym for pestilence, which refers nonspecifically to any acute epidemic accompanied by high mortality. But the term also refers to the recurrent waves of bubonic plague punctuating European history from 1348 to 1720. Bubonic plague epidemics occurred when Yersinia pestis, a rodent disease, was communicated to humans through the bite of infected fleas. Humans have exceedingly poor immune defenses to this organism, and within 6 days of infection most victims develop a grossly swollen lymph node, a bubo, signifying the body’s attempt to contain and arrest multiplication of Y. pestis. On the average, around 60 percent of those infected died within a week after the appearance of the bubo. Thus bubonic plague brought high and dramatic rates of mortality when it extended into human communities.

Distribution and Incidence

With the historically ironic exception of western Europe, Y. pestis today occurs naturally throughout the world among the wide variety of rodents and lagomorphs (i.e., rabbits and related species). Some of the more than 300 rodent species affected are relatively resistant to disease from Y. pestis and can survive and reproduce while technically infected by the organism. Y. pestis infects new animals either because fleas transmit it or because the microbe is shed and survives in the protective microclimate of warm rodent burrows. Some literature refers to this part of the plague cycle as “sylvatic” plague, or “enzootic” plague.