Plague is rare and treatable with antibiotics if caught early.

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Authorities in Oregon have confirmed a case of the bubonic plague in a teenager who was believed to have contracted the disease from a flea bite.

Plague is rare and treatable with antibiotics if caught early, but federal authorities have been puzzled by an increase in cases this year.

Health officials in Oregon said they thought the girl was infected during a hunting trip Oct. 16 near Heppner, in the foothills of the Blue Mountains in the northeastern part of the state.

She fell ill Oct. 21 and was hospitalized days later. The unidentified teen remains in the intensive-care unit.

There have been no other reported recent cases.

Plague is an infectious bacterial disease that is carried by wild rodents and transmitted to their fleas, which then carry the infection to other animals or humans through bites. Symptoms include fever, chills, headache, weakness and a cough.

Bubonic plague, the most common form of the disease, affects the lymph nodes. Two other types of plague are septicemic, a blood infection, and the most contagious form, pneumonic, which infects the lungs. It is not transmitted from human to human unless the patient also has a lung infection and is coughing.

During the 14th century, the plague, also known as the Black Death because of the symptom of oozing, blackened sores, killed tens of millions of people in Europe, Asia and Africa. An estimated 25 percent to 60 percent of the population of Europe — some 50 million people — died and some histories put the toll as high as 200 million throughout the world over the century.

Antibiotics can beat all forms of plague if an infection is caught early. Untreated, it is fatal in 66 percent to 93 percent of cases. With treatment, mortality has been reduced to about 16 percent, according to the Centers for Disease Control and Prevention (CDC). There is no vaccine.

In recent decades, an average seven human plague cases have been reported each year, according to the CDC. Since April 1, there have been at least 11 U.S. cases in humans, three of them fatal, affecting residents of Arizona, California, Colorado, Georgia, New Mexico and Oregon, the CDC said in August.

“It is unclear why the number of cases in 2015 is higher than usual,” the CDC said.

Two of the reported cases were linked to Yosemite National Park.