Burning plague-spreaders. Medieval woodcut of plague-spreaders being executed by burning at the stake. After the Black Death in the 14th century, when between a third and a half of Europe's popu- lation died, plague remained endemic for over 300 years. During the occasional outbreaks anyone accused of spreading the disease was dealt with summarily. The absurdity of the evidence upon which such accusations were based made little difference: the merest suspicion was enough to excite a frenzy of revenge. The bubonic plague was actually spread by rat fleas carrying the bacteria Yersinia pestis, although the related pneumonic" plague could be spread from person to person."