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

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

Septicemic plague occurs if bubonic plague is left untreated, or it can occur directly if the disease enters the bloodstream. Septicemic plague causes bleeding into the organs. The name comes from septicemia, which refers to a serious blood infection.