How does pneumonic plague affect the lungs?

In pneumonic plague, the lungs are extensively invaded by the bacterium Yersinia pestis. This infection leads to severe pneumonia, characterized by fever, weakness, and shortness of breath.1

As the disease progresses, the lungs experience edema, which is the filling of the lungs with fluid. This condition is highly dangerous; without prompt antibiotic treatment, it is almost certainly fatal within three to four days. Unlike other forms of plague, the pneumonic form is highly infectious, as the bacteria can be transmitted to other people through droplets expelled during coughing or sneezing.1