Yersinia pestis is the bacterium that causes plague, including bubonic plague spread historically by infected fleas.
The bacterium cycles naturally among wild rodents and their fleas. An infected flea can transmit Y. pestis when it feeds on another mammal, including a human. Bubonic plague typically causes fever and painful swollen lymph nodes called buboes. Untreated infection can progress to septicemic or pneumonic plague.
Y. pestis caused several major historical epidemics, including the fourteenth-century Black Death. Ancient DNA studies have detected its genetic signatures in victims of that pandemic, linking the historical disease to the modern species.
Plague is now uncommon in most places but has not disappeared. It remains a zoonotic disease in parts of Africa, Asia and the Americas. Prompt antibiotic treatment can be effective, while pneumonic plague is especially concerning because respiratory droplets can permit person-to-person spread.