Anteroposterior chest x-ray of a plague patient showing bilateral pulmonary infection. Yersinia pestis is the bacterium that causes bubonic plague (the black death of the middle ages). The bacterium is primarily a flea-carried pathogen of rats. Infection is rapid, causing swelling in the lymph nodes and leading to septicemia and pulmonary infection. Extensive control measures, directed against the rats as well as their fleas, have essentially banished the plague from Europe, but there are still many regions of the world where the disease occurs.