pandemics
A pandemic is the spread of a disease over a whole country or the whole globe. The flu pandemic of 1918 was the worst in history, infecting about a third of the world's overall population at the time and resulting in the deaths of an estimated 20 to 50 million people.
In the years since the 1918 flu pandemic, no other disease outbreak has killed so many ... until the HIV/AIDS pandemic that began in 1981. Since 1981, AIDS has resulted in the deaths of about 36 million people. But that's over several decades of time, whereas the flu pandemic in the early 20th century killed that many (or more) in just a year's time.