It is said that harmful alleles are eliminated from a population over a period of time,yet sickle cell anaemia persists in the human population. Why?

(N/A) Harmful alleles are typically eliminated from a population over time due to natural selection. However,sickle cell anaemia $(SCA)$ persists because it provides a selective advantage in regions where malaria is endemic. Individuals with the sickle cell trait (heterozygous carriers) possess a significant resistance to malarial infection. When the malaria parasite $(Plasmodium)$ enters the red blood cells,it causes a drop in oxygen levels. In carriers,this reduction is sufficient to trigger the sickling of red blood cells,which leads to the rapid removal of these infected cells from circulation by the spleen. This process effectively limits the progression of the malarial infection. Consequently,these individuals have a higher survival rate during malarial outbreaks,ensuring the $SCA$ allele is maintained in the gene pool through balancing selection.