Sickle cell anemia is caused due the production of abnormal hemoglobin proteins by the person affected by it. There are two genes which result in this trait. A person who has inherited both the genes for sickle cell anemia, one from each parent has abnormal red blood cells which have a reduced capacity to carry oxygen and this inhibits normal daily activities of the person.
In case a person has inherited only one gene for sickle cell anemia from a parent and the other gene is normal, the person suffers from only a slight impediment. But the resulting defect in the protein, protects the person from acquiring malaria and dying due to the illness. In areas in Africa and Asia where malaria is rampant, the presence of just one defective gene for sickle cell anemia allowed people to live a relatively normal life and as they were protected from malaria their offspring were also carriers of sickle cell anemia.
This is a very good example of natural selection resulting in an increase in the number of people who have a particular gene in the subsequent generations as it increases their chances of survival.