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]).
The Spanish flu's mortality isn't that mysterious today (samples have been extracted from victims buried in cold climates, and its genome sequenced). It turns out that in young, healthy victims it provokes a cytokine storm -- an overreaction by the immune system that causes a massive inflammatory response and fluid build-up in the lungs (with potential for secondary bacterial infection). Ironically, older or younger victims with weaker immune systems were less likely to succumb to this reaction:
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...