Xenopsylla cheopis (oriental rat flea) with plague mass in poventriculus with finger-like projection into stomach, third day of infection with P. pestis. This flea is the primary vector of plague in most large plague epidemics in Asia, Africa, and South America. Both male and female fleas can transmit the infection. Yersinia pestis is the bacterium that causes bubonic plague (the black death of the middle ages), 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.