Yersinia pestis: The bacteria that causes 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."
Y. pestis mainly infects rats and other rodents which are the prime reservoir for the bacteria. Fleas are the prime vectors carrying the bacteria from one species to another. They bite rodents infected with Y. pestis, then they bite people and so transmit the disease to them.
Transmission of the plague to people can also occur from eating infected animals such as squirrels. Once someone has the plague, they can transmit it to another person via aerosol droplets.
Plague occurs in the U.S. It is treatable with antibiotics but, if not treated promptly, can promptly lead to death.
Yersinia is named after the Swiss bacteriologist Alexandre-Emile-Jean Yersin (1863-1943) who identified it in 1894 after a trip to Hong Kong looking for the agent that was killing thousands of people in southern China. The bacteria was also discovered at the same time by the Japanese bacteriologist Shibasaburo Kitasako.