Plague is an infectious disease of animals and humans caused by a
bacterium. People usually get plague from being bitten by a rodent flea
that is carrying the plague bacterium, or by handling an infected
animal. Plague is characterized by periodic disease outbreaks in rodent
populations, some of which have a high death rate. During these
outbreaks, hungry infected fleas that have lost their normal hosts seek
other sources of blood, thus increasing the risk to humans and other
animals frequenting the area.
Rodents, such as rock squirrels, prairie dogs, wood rats, chipmunks other ground squirrels and fleas are the most frequent sources of human infection in the southwestern states. Other less frequent sources of infection include wild rabbits and wild carnivores that pick up their infections from wild rodent outbreaks. Domestic cats (and sometimes dogs) are readily infected by fleas or from eating infected wild rodents. Cats may serve as a source of infection to persons exposed to them. Pets may also bring plague-infected fleas into the home.
Plague is transmitted from animal to animal and from animal to human by the bites of infected fleas. Plague is also transmitted by inhaling infected droplets expelled by coughing, by a person or animal, especially domestic cats, with pneumonic plague. Transmission of plague from person to person is uncommon and has not been observed in the United States since 1924.