Which bacterium causes plague, the disease associated with the Black Death?

Yersinia pestis is the bacterium that causes plague, including the disease associated with the Black Death.

The bacterium is a Gram-negative member of the Enterobacteriaceae family. It is maintained in animal populations, especially rodents, and can spread to people through the bites of infected fleas. The oriental rat flea, Xenopsylla cheopis, played an important role in historical outbreaks.

Plague has three main clinical forms: bubonic, septicemic, and pneumonic plague. Bubonic plague causes swollen, painful lymph nodes called buboes. Pneumonic plague affects the lungs and can spread directly between people through respiratory droplets.

The Black Death reached Europe in the fourteenth century and caused enormous mortality, but historians continue to study its exact transmission patterns and social effects. Modern antibiotics can treat plague effectively when given promptly. Yersinia pestis should not be confused with Bacillus anthracis, which causes anthrax, or Salmonella Typhi, which causes typhoid fever.