1918-1919, a fifth of the worlds population was infected with influenza. This pandemic killed 675K Americans, 10 times more than died in WWI (over 20M worldwide).
Malaria now infects some 300 million people each year, killing at least 2 million children and adults. Two children die of malaria every minute.
The World Health Organization estimated that, worldwide, between 2.8 and 3.5 million people with AIDS died in 2004.