Yersinia pestis, Colored transmission electron micrograph (TEM) of Yersinia pestis, the bacterium which causes bubonic plague (the Black Death of the Middle Ages). Here, two recently divided bacteria are seen still attached. They are rod-shaped, Gram-negative and non-motile bacilli. Yersinia pestis is primarily carried by the fleas of rats. Transfer to humans occurs when such fleas feed on human blood. Infection is rapid, causing swollen lymph nodes, and leading to septicaemia & pulmonary infection. Extensive control measures against rats & their fleas have eliminated plague from Europe, but it occurs in other regions of the world. Mag: x12,000 at 6x4.5 size.