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.
Read MoreAlzheimer's worsens twice as fast in women, study finds
"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."