False colour transmission electron micrograph (SEM) of Yersinia pestis, a species of Gram negative, non motile, capsulated, aerobic bacilli (ovalrod shaped bacteria) that is the causative agent of bubonic plague (the Black Death of the Middle Ages) in humans. The bacteria are primarily flea carried pathogens of rats. Transfer to man occurs when a flea is obliged to leave its dead rodent host (often because its feeding apparatus is blocked by multiplying bacteria and it is starving) and suck human blood. Infection is rapid, featuring swelling (bubo) of lymph nodes and leading to septicaemia and pulmonary infection. Magnification: X 10,500 at 35mm size.