Plague bacteria. Coloured scanning electron micrograph (SEM) of Yersinia pestis bacteria. This species is the cause of bubonic plague (generally agreed to have been the Black Death of the Middle Ages). The bacteria are rod-shaped, Gram- negative, non-motile bacilli. Yersinia pestis is primarily carried by the 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. Magnification unknown.