The bubonic plague, or Black Death, emerged in the 14th century. Originally spreading to humans from rats, the pandemic killed roughly 200 million people, about half of Europe's population. It took over 200 years for the continent's population to recover. This places Black Death as the deadliest pandemic in history by a significant margin - second place belonging to smallpox, which killed 50 million people. The disease was also terrifyingly contagious and deadly. Simply touching a sick person was said to be enough to infect someone else, and people who were healthy in the evening could be dead by the morning. The Black Death was unprecedented in its deadliness, in a way that certainly has not been seen since.
[P1] The Black Death killed many more people than any other pandemic. [P2] It is therefore by far the worst pandemic.