Y. pestis is found in rodents and fleas. Can cause plague in humans.
About Yersinia Pestis
Plague is an infectious disease caused by bacteria called Yersinia pestis. These bacteria are found mainly in rodents, particularly rats, and in the fleas that feed on them. Other animals and humans usually contract the bacteria from rodent or flea bites. NIH – National Institute of Allergy and Infectious Diseases