The Black Death was one of the deadliest pandemics in human history, peaking in Europe between 1348 and 1350. It is widely thought to have been an outbreak of bubonic plague caused by the bacterium Yersinia pestis, but this view has recently been challenged. Usually thought to have started in Central Asia, it had reached the Crimea by 1346. From there, probably carried by fleas residing on the black rats that were regular passengers on merchant ships, it spread throughout the Mediterranean and Europe.
The Black Death is estimated to have killed 30% to 60% of Europe's population, reducing the world's population from an estimated 450 million to between 350 and 375 million in 1400. This has been seen as creating a series of religious, social and economic upheavals which had profound effects on the course of European history. It took 150 years for Europe's population to recover. The plague returned at various times, resulting in a larger number of deaths, until it left Europe in the 19th century.