As mentioned in another comment, smallpox has killed more than it's fair share of humans. An estimated 300 million in the 20th century alone[0].

Maybe lesser known is the Spanish Flu which killed more than 50 million people worldwide within 2 years (1918, 1919). The strangest thing is that it seems to have largely targeted young and healthy adults (it, like the Black Plague, seems to have originated in China[1]).

[0] -http://www.who.int/about/bugs_drugs_smoke_chapter_1_smallpox... [1] - http://news.nationalgeographic.com/news/2014/01/140123-spani...