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Plague bacteria (Yersinia pestis), coloured scanning electron micrograph (SEM). These are Gram negative non-motile bacilli (rod-shaped bacteria). Y. pestis causes bubonic plague, thought to be the Black Death of Europe in the mid-14th century, and also the Great Plague of London in 1664-1665. The bacteria are spread to humans by bites from infected fleas carried on rats. Infection is rapid, causing swollen lymph nodes and leading to septicaemia, which is always fatal, and pulmonary infection, which can be fatal within a day. Prompt treatment with antibiotics can effect a cure. Magnification: x5000 when printed at 10 centimetres tall. (KEYSTONE/SCIENCE PHOTO LIBRARY/EYE OF SCIENCE/SCIENCE PHOTO LIBRARY)