Bubonic plague infection causes tiny blood vessels in the hands and fingers to clog up and cut off circulation. Without blood, the flesh dies and turns black (called "gangrene"). This is why in the Middle Ages bubonic plague was called "the Black Death." In the 14th century it killed an estimated 25 million people, or 30–60% of the European population.
51 Historical Facts That Are Totally Messed Up | 8. After Pope Gregory IX associated cats with devil worship, cats throughout Europe were exterminated in droves. 9. This sudden lack of cats lead to the spread of disease because infected rats ran free. The most devastating of these diseases, The Bubonic Plague, killed 100 million people.