Yersinia pestis: Clinical Forms and Medieval Medicine

The bacterium responsible for the Black Death, Yersinia pestis, was identified in 1894 by Alexandre Yersin and Kitasato Shibasaburō during an outbreak in Hong Kong, and its role in the medieval pandemic was confirmed definitively through ancient-DNA studies extracted from the dental remains of victims buried in fourteenth-century plague cemeteries such as East Smithfield in London. The disease took three clinical forms. Bubonic plague, the most common, followed the bite of an infected flea and produced painful, swollen lymph nodes known as buboes, typically in the groin, armpit, or neck, with an untreated mortality rate estimated between 30 and 60 percent. Septicemic plague, in which the bacterium invaded the bloodstream directly, caused dark subcutaneous blotches and carried a mortality rate close to 100 percent; these dark blotches are the probable origin of the later term "Black Death," even though medieval contemporaries themselves generally spoke simply of "the great mortality." Pneumonic plague, the most lethal and contagious form, spread directly between people through respiratory droplets, required no fleas or rodents as intermediaries, and killed within days.