How can bubonic plague develop into pneumonic plague?

Bubonic plague can progress into pneumonic plague when the Yersinia pestis bacteria, which initially colonize the lymph nodes and cause the characteristic swellings known as buboes, spread from the lymphatic system into the bloodstream and subsequently infect the lungs.12

This transition typically occurs as a complication of untreated bubonic plague. Once the lungs are extensively invaded, the condition becomes highly dangerous and is characterized by severe pneumonia, including symptoms such as fever, weakness, and shortness of breath. Unlike the bubonic form, which is transmitted primarily through flea bites, pneumonic plague is highly infectious and can be spread directly from person to person through respiratory droplets expelled by coughing or sneezing. Prompt antibiotic treatment is essential to prevent this progression and to save the patient's life.12