what is sickle cell anaemia ? why has natural selection not eliminated sickle cell anaemia?

Sickle cell Anemia is a hereditary blood disorder in which sickle shaped (crescent shape) red blood cells are formed. These sickle cells tend to block the blood vessels very easily due to their abnormal shape. People inherit this disease from their parents. It is assumed that originally this unusual shape of RBC may have occurred due to some gene mutations.
Natural selection could not eliminate sickle cell anemia as not everyone who carries sickle cell gene suffers from sickle cell anemia. Some of them only carry the gene and may be in next generation the disease occurs.