A person in Arizona recently died of pneumonic plague—a rare and severe form of the disease. An expert explains how the bacteria that spurred the Black Death centuries ago continues to claim lives

Pneumonic plague is the most serious form of plague that is caused when bacteria spread to the lungs of a patient with bubonic or septicemic plague.

The pneumonic plague killed a US resident on Friday, marking the first fatality from the infection since 2007. The US officials said that the death of the Arizona resident was the first known incident in 18 years as the last death was reported in 2007,

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.