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

Explanation

Sickle-cell anaemia is an autosomal recessive disase caused by haemoglobins an oxygen carrying protein in blood cells.
Despite the disease's lethal symptoms, it protects the carrier from malaria. Its allele are most common in the people of African descent (about 7% people of African descent carry an allele) and some other are as where malaria in prevalent.
It provides the vital protection form malara. Individuals with HbAS heterzygotes rend to survive better than individuals with HbSS (homozygotes) as they are not exposed to the same severity of risk.