What were the greatest epidemics on earth?

Pandemic. The world's worst Pandemics were: The Antonine Plague 165 to 180, The Plague of Justinian 541, The Black Death (Plague) 1347 to 1351, 1830 - 1851 Cholera Pandemic, 1918 Spanish Flu Pandemic, 1957 Asian Flu Pandemic, HIV/AIDS,
Probably many. Two that come to mind are the great plaque in medieval europe which killed 100's of thousands and the flu epidemic of 1918 which did in 1-2 million. There are others of greater or lesser extent, and many seasonal diseases which continue as well as disease like malaria which is not an epidemic but kills tens of thousands steadily in modern times.