The Black Death was a devastating pandemic that originated in China in the 1330s and spread to Europe. It was caused by the bacterium Yersinia pestis, which was carried and transmitted to humans by fleas on infected rats. There were three main forms: bubonic plague which caused swollen lymph nodes, pneumonic plague which infected the lungs, and septicemic plague which caused high fevers and purple skin discoloration. All forms had high mortality rates. The plague arrived in Europe in 1347 via infected merchant ships from the Black Sea docking in Sicily, and then spread rapidly across the continent killing around 25 million people, one third of Europe's population.