The Spanish Flu began in later 1918 and is responsible for about 1/3rd of all military casualties (or deaths? It's been a while, paging /u/NMW) and killed about 5% of the world population as a whole.

From American War and Military Operations Casualties: List and Statistics here, it notes:

World War II was the first war in which there were more battle casualties than deaths from other causes, such as accidents, disease, and infections.

Battlefield medicine advances came a long way after WWI - technology such as penicillin also became mass produced during WW2 which also staved off a lot of infections.

Note too that the statistics prior to recent wars don't break down battle deaths and non-combat deaths as clearly as recent statistics, which includes hostile vs. non-hostile actions (including friendly fire).

Note too that accident rates, per # served and in raw totals, has gone down considerably in recent decades. An aviation accident of a B-17 bomber in WW2 may take down 10 crewmembers - today, single-seat fighter jets carry twice as many bombs as a B-17 so far fewer people and planes are required to do the same job. Not to mention, safety procedures implemented by the military have reduced the rate of accidents considerably since the 1950s.