Plague of Justinian 541-542 100,000,000 (25-50M)
Black Plague 1346-1350 50,000,000 (75-200M)
1918 Flu (Spanish Flu) 1918-1920 20,000,000 (50-100M)
[0] "Estimates believe 100 million people died during this time which was half the world population."
[1] https://en.wikipedia.org/wiki/Plague_of_Justinian
[2] "Some historians believe the plague of Justinian was one of the deadliest pandemics in history, resulting in the deaths of an estimated 25–50 million people during two centuries of recurrence, a death toll equivalent to 13–26% of the world's population at the time of the first outbreak."