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What is the plague?
01:25 - Source: CNN

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CDC: Four people have died among 15 diagnosed with plague in U.S. this year

Reason for the higher than usual number of cases this year is unknown

Atlanta CNN  — 

Fifteen people have been infected with bubonic plague so far this year in the United States, the Centers for Disease Control and Prevention told CNN. Four of those cases were fatal.

That’s four more cases and one more death than the CDC previously reported.

On average, there are seven cases per year, although in 2006 there were 17. An August report from the CDC put health care providers on notice to consider the plague in patients with symptoms and a travel history to an area where the plague has been reported.

Plague occurs in rural and semirural areas of the Western United States, most commonly New Mexico, Arizona and Colorado. This year’s cases have been reported in eight states.

Related: How do we still have the plague, centuries after the Black Death?

There have been two patients in Arizona, four in Colorado, four in New Mexico, one in Oregon, one in California, one in Utah, one in Georgia and one in Michigan. The patients in Georgia and Michigan were infected in California and Colorado, respectively.

The plague usually occurs from late spring to early fall but can be found any time of the year. There could still be more cases diagnosed this year, according to Dr. Natalie Kwit, a veterinarian with the division of vector borne diseases at the CDC.

“If rodents aren’t out and about and humans aren’t, there will be no exposure,” she said.

A reason for the higher number of cases this year is not known, according to Kwit. In the meantime, she says that the number of “cases does tend to fluctuate due to interaction between fleas, the rodents they are on, and what the humans are doing.”

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 usually become infected from fleas that have fed off of an infected rodent such as a rat, squirrel or chipmunk.

Symptoms in humans develop between two and six days after exposure to the bacteria. They include an overall feeling of sickness, sudden fever, abdominal pain, swollen lymph nodes, nausea and vomiting.

Related: How they survived bubonic plague

The plague can be successfully treated with antibiotics, but only with a prompt diagnosis and treatment. It’s important to get to the doctor as soon as possible if you are experiencing symptoms and have been in an area where the plague has been found. The death rate is 16% among patients who have been treated and between 66% and 93% among those who are not treated, according to the CDC.

It recommends individuals in areas where the plague is found to protect themselves by wearing long pants and using insect repellent on clothing and skin. The CDC also recommends protecting pets from fleas and removing anything that can be a food source for rodents, such as garbage, clutter and brush.