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.