Sickle cell anemia is a genetic disorder whose gene likely originated in places where illnesses like malaria are common; the sickle cells help prevent people from getting sickened as bad in that case.
With less oxygen, the abnormal hemoglobin S gene can cause rigid, nonliquid protein strands to form within the red blood cell. These rigid strands can change the shape of the cell, causing the sickled red blood cell that gives the disease its name.