I think if you search around you will find you are correct! Random results of chasing after the answer showed me some stuff!

https://www.who.int/news-room/fact-sheets/detail/the-top-10-causes-of-death

Clearly the work done in getting safe water has been way more effective than I was paying attention to. My statement was correct at one point, IIRC. It stuck with me, and you asking made me check! I may have read a statistic on causes of death of children. Either way, I am glad you asked!

War has always been pretty ineffective at killing people, though the people it kills are the engines that drove countries, historically: young males. And the concomitant civil unrest causes serious excess deaths. The Iraq war "only" killed 4500 US troops, and 15,000 Iraqi forces, but total excess deaths number as high as 1 million.

WWI had total military deaths at 10 million, but the resultant spread of Spanish flu caused at least 50 million deaths, and maybe as much as 100 million, in a world population of 1.5 billion.

Epidemics were the only truly effective killers, outside of China, which has had some insanely deadly civil wars, but the big one, The Taiping Rebellion, killed most people from the resultant famine from loss of central government control over irrigation and flood control. (30 million deaths out of a population of 450 million.)

Justinian's Plague,

>The Justinian plague in the sixth century and is estimated to have killed between 30 and 50 million people—about half the world's population at that time—as it spread across Asia, North Africa, Arabia, and Europe.

Black Death >The Black Death was the second great natural disaster to strike Europe during the Late Middle Ages (the first one being the Great Famine of 1315–1317) and is estimated to have killed 30 percent to 60 percent of the European population, as well as about one-third of the population of the Middle East.[14][15][16] The plague might have reduced the world population from c. 475 million to 350–375 million in the 14th century

and Spanish Flu all killed a significant percentage of the world's population.

Of course the 20th century managed to make men capable to serious mechanized death. Hitler, Stalin, and Mao all killed a serious percentage of their own countries populations.

TL;DR Justinian's Plague killed half the world's population!