Even though the common reservoir host for the plague bacterium is usually rodents, it is the infector (fleas) that actually transmits it between hosts. Plague can be transmitted to humans via contact with or biting of infected fleas, as well as contact with infected mammals. No confirmed case of human-to-human plague has been documented in the United States since 1925.
Over 200 mammalian species have been documented contracting plague, including the Montana Species of Concern Black-tailed Prairie Dog, White-tailed Prairie Dog and Black-footed Ferret (PHAC 2001).