Y. pestis mainly infects rats and other rodents. Rodents are the prime reservoir for the bacteria. Fleas function as the prime vectors carrying the bacteria from one species to another. The fleas bite the rodents infected with Y. pestis and 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 (e.g., in the southeastern U.S.) Once someone has the plague, they can transmit it to another person via aerosol droplets.
History -- Yersinia is named after a 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 same discovery was made at the same time by a Japanese bacteriologist Shibasaburo Kitasako.
The plague has been responsible for devastating epidemics. The disease occurs endemically (at a consistent but low level) in many countries including the United States. "La Peste" (The Plague), a novel (1947) by the Nobel Prize-winning French writer Albert Camus (1913-1960) is set in the Algerian city of Oran overrun by a deadly epidemic of the plague.
Bioterrorism -- The General Accounting Office, the investigative arm of the US Congress, in a 1999 report considered plague to be a "possible, but not likely" biologic threat for terrorism, as it is difficult to acquire a suitable strain of Y. pestis and to weaponize and distribute it. Seed stock is difficult to acquire and to process and heat, disinfectants and sunlight render it harmless.
The plague is also known as pest and pestis.