Terrifying.
According to the NLM-NIH
"Bubonic plague, an infection of the lymph nodes
Pneumonic plague, an infection of the lungs
Septicemic plague, an infection of the blood.
Septicemic plague may cause death even before its symptoms occur.
Without treatment, about 50% of persons with bubonic plague die. Almost everyone with pneumonic plague dies if not treated. Treatment reduces the death rate to 50%."