1918 Influenza (Spanish flu) killed over 50 million when world population was about 2 billion. Don’t know the actual or estimated rate. This seemed to last nearly 3 years.
More at WIKI.
There is a cemetery in west-central Texas near our family ranch. There are many headstones there with death dates in 1918. Most are infants and toddlers. It’s very sobering to walk through them.