Smallpox (430 BC? - 1979):Killed more than 300 million people worldwide in the 20th century alone, and most of the native inhabitants of the Americas 
Spanish Flu (1918 - 1919):Killed 50 to 100 million people worldwide in less than 2 years

Black Death (1340 - 1771):Killed 75 million people worldwide
Malaria (1600 - today):Kills about 2 million people per year
AIDS (1981 - today):Killed 25 million people worldwide
Cholera (1817 - today):8 pandemics; hundreds of thousands killed worldwide
Typhus (430 BC? - today):Killed 3 million people between 1918 and 1922 alone, and most of Napoleon's soldiers on Russia