It is impossible for the human tongue to recount the awful truth. The victims died almost immediately. They would swell beneath the armpits and in the groin and fall over while talking. Father abandoned child, wife husband, one brother another; for this illness seemed to strike through breath and sight In many places great pits were dug and piled deep with the multitude of dead. And they died by the hundreds, both day and night

Agnolo di Tura chronicled this tragic yet common occurrence in Siena, Italy, in 1348. Called the Great Pestilence, the disease was later known as the Black Death or bubonic plague, and it swept through Europe with frightening speed. Spread by the bite of an infected flea, the Yersinia pestis bacteria caused the most devastation recorded in human history. Within four years it killed one-third of the population of Europe, more than 25 million...