Light micrograph of the rat flea Xenopsylla cheopis. The rat flea plays a major role in the spread of Yersinia pestis, the bacterium which causes bubonic plague (the Black Death of the Middle Ages). The bacterium is carried by the rat flea, which in turn infests the domestic and brown rats which live in close association with humans. 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 and, at intervals, breaks out in an epidemic.