What was the worst pandemic in history?

There are two prime contenders for the title of the worst pandemic in human history - The Bubonic Plague of 14th Century Europe and the Spanish Flu Pandemic of 1918. In terms of incidence and number of deaths, the Spanish Flu Pandemic dwarfs The Bubonic Plague. The Spanish Flu Pandemic is estimated to have affected roughly 500 million people and caused 50 million to 100 million deaths worldwide between 1918 and 1920. The Bubonic Plague killed approximately 30 million people, or 30 percent of the population of Europe, between 1346 and 1351 - far fewer fatalities than attributed to the Spanish Flu. However, in a time when virtually no knowledge of modern infectious disease theory or treatment existed, this pandemic had far reaching social, political, and economic effects. Among these effects include the rise of new socioeconomic classes and the establishment of new sociopolitical and economic European power centers.