Malaria, a disease the infects RBCs, and sickle cell anemia (SCA), a homozygous recessive disease that shortens the life cycle of RBCs, both effect millions of people worldwide. During massive malarial outbreaks, researchers have observed that individuals with a mild form of SCA are more disease resistant when compared to normal individuals within the same population. Which of the following accounts for this finding? (QID 133403)