Darwin, Northern Australia has had a white farmer presence for over a century, and it's at a comparable latitude to Nigeria.
Did you not hear the word "medieval"? And white farmers had the advantage of some degree of medical care in that region. Even a hundred years ago people had an idea of how to eliminate things like mosquitos. I'm not sure white people in Nigeria or the Congo will do so well, especially if they are treated in a way comparable to the way blacks were treated in the Antebellum South.