Doctors should think about the possibility of plague when they see patients with flu-like symptoms this summer and should ask if they've been camping, hiking or around dead rodents, federal health officials said Tuesday.
Eleven cases of plague have been recorded in the U.S. this year, and three people have died.
"It is unclear why the number of cases in 2015 is higher than usual," the Centers for Disease Control and Prevention says in a brief update.
The CDC has recorded 91 cases of plague since 2000, an average of between 5 and 6 a year. The highest number this century was 17 cases in 2006. Now, 2015 is the year with the second-highest number of cases.
In a country of more than 300 million people, plague is still extremely rare. But it's easy to mistake a plague infection for a virus, and doctors usually have no medication to prescribe for a viral infection. Plague is easy to treat if caught in time - antibiotics will take care of it - so doctors should think about whether a patient might have it.
"Health care providers should consider the diagnosis of plague in any patient with compatible signs or symptoms, residence or travel in the western United States, and recent proximity to rodent habitats or direct contact with rodents or ill domestic animals," the CDC says in its report.
"In humans, plague is characterized by the sudden onset of fever and malaise, which can be accompanied by abdominal pain, nausea, and vomiting."
Suspicious doctors can send samples to state health departments to be tested.
Two of the cases this year were traced to either Yosemite National Park in California or the nearby Sierra Nevada Mountains. Park officials closed two camp areas to spray for fleas there.
"Plague circulates among wild rodents and their fleas in rural and semirural areas in the western United States," the CDC noted.
As during the epidemics of plague that killed off huge numbers of people in Europe and Asia in centuries past, plague is carried by fleas. People can catch it by feeding or handling rodents such as squirrels or from pets who have caught fleas.