The average number of cases between 2001 and 2012 was seven, with less than one death each year.
"We don't want people to panic but we do want people to be aware of the heightened risk," said Dr. Natalie Kwit, a veterinarian with the division of vector borne diseases at the CDC.
The cases, which are required to be reported to the CDC, have been reported in six states. There have been two cases in Arizona, one in California, four in Colorado
, one in Georgia, two in New Mexico and one in Oregon. The cases in California and Georgia have been linked to areas in or near Yosemite National Park
in the southern Sierra Nevada, the report says.
The youngest of the patients is 14 and the oldest is 79. Nine of the patients were male.
Plague occurs in rural and semi-rural areas of the Western United States, most commonly in New Mexico, Arizona and Colorado.
The bacteria that causes the plague, Yersinia pestis, is naturally occurring in the environment and is found in areas where there are wild rodents.
People are usually infected after a flea becomes infected from a rodent such as a rat, squirrel or chipmunk, and then transmits it to a person by biting them.
Patients usually develop symptoms between two and six days after exposure. They include generally feeling sick, a sudden fever, abdominal pain, nausea and vomiting.
Patients can be successfully treated with antibiotics but only if they are diagnosed and begin treatment early. In patients who aren't treated, the death rate is
between 66% and 93%, compared to 16% among those who are treated.
Cases can occur year-round but are likely to appear from late spring to early fall. Kwit said it's hard to say whether there will be more cases this year.
Kwit said it is unclear why the number of cases is higher than usual but added that the number tends to fluctuate. "This may be one of those years where we are seeing a higher number [of cases]," she said, noting that in 2006 there were 17 cases.
Kwit said the point of the report is to put the plague on the radar for physicians who may see patients with a fever or swollen lymph nodes who have a travel history and exposure history.
There are steps people can take to protect themselves in areas where the plague is found. The CDC recommends wearing long pants and using insect repellent on clothing and skin. It also recommends protecting pets from fleas and removing garbage, clutter, brush and anything that could be a food source for rodents.