Yersinia pestis

Yersinia pestis is a Gram-negative facultative anaerobic bipolar-staining (giving it a safety pin appearance) bacillus bacterium belonging to the family Enterobacteriaceae. The infectious agent of bubonic plague, Y. pestis infection can also cause pneumonic and septicemic plague. All three forms have been responsible for high mortality rates in epidemics throughout human history, including the  Black Death that accounted for the death of approximately one-third of the European population in 1347 to 1353.
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Yersinia pes·tis n. A bacterium that causes plague and is transmitted from rats to humans by the rat flea Xenopsylla cheopis. [more]Yersinia pestis - Community and Resources

Noun
1. a Bacillus bacteria that causes plague; aerosolized bacteria can be used as a bioweapon
(hypernym) Bacillus, Bacilli, B

The bacterial cause of the bubonic plague which in the year 541 (as the Black Death) and later in the Middle Ages decimated Europe. The effects of the plague are described in the nursery rhyme "We all fall down." It is transmitted to humans by the bite of fleas that have fed on infected animals, mostly rodents. Plague occurs in the U.S. It is treatable with antibiotics but, if not treated promptly, can promptly lead to death.