Sickle cell anaemia has not been eliminated from the African population because:

Sickle cell anaemia in which RBCs become sickle-shaped and stiff is a genetic disorder which is autosomal and linked to a recessive allele. It has not been eliminated from the African population because it provides immunity against malaria. People who are heterozygous for the sickle cell allele are much less susceptible to falciparum malariaone of the main causes of illness and death. Since sickle-shaped RBC effectively kills the malaria parasite, the individuals heterozygous for this variant are able to cope with malaria infections much better than normal persons. The natural selection thus maintains this variant form of haemoglobin along with the normal haemoglobin in malaria prone areas.