What role does natural selection play in the spread of sickle cell anemia?

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.