Plague bacteria. Petri dish containing cultured colonies of Yersinia pestis bacteria. This bacterium is the cause of bubonic plague (generally agreed to have been the Black Death of the Middle Ages). Y. pestis is primarily carried by fleas on rats. Transfer to humans occurs when the fleas feed on human blood. Infection is rapid, causing swollen lymph nodes, and leading to septicaemia and pulmonary infection. Extensive control measures against rats and their fleas have eliminated plague from Europe, but it still occurs in other regions of the world. These bacteria have been cultured in a general purpose nutrient medium called tryptic soy agar (TSA).