The plasmodium of the Malarial parasite cannot thrive in sickle cells. It is now believed that Sickle Cell Anemia is one of a set of diseases that, with one gene for it expressed, only weakens the host while protecting against something else, in this case, malaria. But with TWO genes expressed, the host cannot easily survive without strong treatment. Map distributions are largely overlapping between the two populations of malaria and sickle cell.
If it was a perfect defence against Malaria, perhaps the parasite would have occasion to mutate and find a way around the defence.
Resources for learning more about Sickle Cell Diseases
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