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.
10 things you (probably) didn’t know about the Black Death - It is one of the worst catastrophes in recorded history – a deadly plague that ravaged communities across Europe, changing forever their social and economic fabric. But how much do you know about the Black Death? | History Extra
The Black Death is far and away the most lethal epidemic in history. The statistics alone are staggering. The Plague’s 13-year reign across the world resulted in at least 75 million deaths, including at least a third of Europe. The pandemic originated as a bubonic plague outbreak in China in the 1330s and was then spread by the fleas of infected rats. Symptoms included fever, headaches, nausea, painful buboes and rashes.