I have posted previously about bubonic plague here and here. The bubonic plague is caused by the bacteria, Yersinia pestis. The bubonic form is transmitted by the rat flea and is the most common form of Yersinia pestis. This form of the plague is present in rodent populations in many desert areas including arid areas of the US.
Yersinia pestis, can spread to the lungs (the pneumonic form of the plague) and is much more rapidly transmitted from person to person by coughing and aerosols. Yersinia pestis is listed as a Class A Bioterrorism agent by the CDC (Center for Disease Control). Discovery of human cases of bubonic plague typically gather a lot of attention and set off investigations into the source.
There are typically only about 10-20 cases per year and that number has been dropping. One factor may be higher nighttime temperatures in areas where plague is prevalent, creating drier conditions in the rodent burrows. Dry conditions lead to higher flea mortality and less transmission and prevalence of plague in rodent populations.
Plague is so rare, it is low on the list of likely suspects. Almost no medical professionals in North America have experience treating cases of plague. One of the most recent cases of plague involved two members of the same household in a desert area of Oregon. The individuals became very sick, but the medical professionals had difficulty diagnosing the cause as Yersinia pestis. The correct diagnosis took over 3 weeks and by that time, fortunately, the patients had recovered. Investigations into the source of the plague found that one of the family pets, a dog had elevated immune response to plague. The best guess is the dog contacted infected rodents or fleas from infected rodents. During close contact with the dog or with fleas on the dog, the Yersinia pestis was transmitted to the patients.
Plague is rare in North America but people need to be cautious when coming in contact with rodents in areas where plague exists. The CDC provides a map of counties where the plague is most frequent.