I am mildly disappointed to learn that this is probably an overestimate, although it is pretty significant http://factmyth.com/factoids/malaria-killed-half-the-people-...

Thanks for calling it out! I think I first heard this from Plasmodium researchers, and in my mind the claim has strengthen over time. A better statement would be: "“Malaria could have potentially killed nearly to half the people who ever lived, predominantly children”."

And this is based off the idea that for most of human history people have lived in Sub-Saharan Africa, where the majority of people who lived died as children, and the predominate cause of death was Malaria.