Why is it that carriers of sickle cell anemia are immune to malaria?

Malaria infects RBCs: Malaria infects red blood cells. Persons with sickle cell trait will often be "immune" because the malaria infection is not allowed to propagate through the rbcs because the sickle trait causes them to die a little faster when infected. Persons with sickle cell disease get very sick with malaria, because of sickle cell related complications. Persons with normal hemoglobin gets sick from malaria.

Malaria and Sickle: Sickle cell disease shortens the life span of a red blood cell. The parasite that causes malaria lives and reproduces in a red blood cell exclusively. The shortened red blood cell life span caused by the sickle cell mutation interferes with the lifecycle of the malaria parasite, thus making the person mostly resistant to malaria.