They have gene selection which reduces the malarial infection rate? Blood disorders which are acute for some carriers of the mutation?
The way it was explained when I was in Med school long ago was “they must get malaria at an early age” and so their immune system hardly reacts to it. People who live in endemic areas don’t get ill. At least, that was the thinking back then. Since we no longer have malaria it hasn’t been a hot topic of my reading since I took the Boards.