Plague bacteria (Yersinia pestis), coloured scanning electron micrograph (SEM). These bacteria cause 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 by bites from infected fleas carried on rats. If untreated it can spread rapidly to the blood causing septicaemia, which is always fatal. It can also spread to the lungs, which can be fatal within a day. Prompt treatment with antibiotics can effect a cure.