Plague is believed to have been introduced to North America from Asia, arriving for the first time about 1900. This was the time of the most recent worldwide outbreak spread from port to port by ship rats. The disease gained a foothold among California ground squirrels in the San Francisco Bay area and during the next 50 years it moved eastward among wild rodents to its present distribution, which included Colorado. It was first seen in this state in San Miguel County in 1941 and appeared in several east-slope locations in 1943. Plague is now firmly established and is frequently detected in rock squirrels, prairie dogs, wood rats, and other species of ground squirrels and chipmunks.
Wild rabbits can also become involved in the plague cycle.
Each rodent species is host to one or more species of fleas which, when infected, are carriers of plague. These fleas generally do not infest other animals unless their natural hosts are unavailable. Their principle flea is an aggressive parasite and will readily bite other animals including people. Domestic cats and dogs can also contract the plague by catching and eating infected rodents and rabbits or by being bitten by infective fleas. They may carry infected fleas home to their owners or, especially with cats, serve as a direct source of infection.