The patient had the pneumonic plague, the rarest and deadliest form of the disease. Human cases remain very uncommon in the United States, though plague is endemic to the western part of the country

Pneumonic plague is both “the least common and most dangerous type of plague,” according to the Cleveland Clinic.

Pneumonic plague is caused by bacteria carried by species like rats and prairie dogs. Pneumonic and bubonic plague transmission and symptoms vary. Pneumonic plague killed a resident of northern Arizona, who died in the emergency room at Flagstaff Medical Center.