Plague bacteria (Yersinia pestis), computer illustration. Y. pestis are gram-negative bacteria from the Enterobacteriaceae family, they have an ovoid shape with bipolar staining (more intensively stained at poles). The bacterium causes bubonic plague (the Black Death of the Middle Ages). The bacterium is primarily a flea-carried pathogen of rats. Transfer to man occurs when a flea is obliged to leave its dead rodent host and feed on human blood. Infection is rapid, causing swellings in the lymph nodes (buboes) and leading to septicaemia 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.