How does the allele that causes sickle cell disease both help and harm people?

The allele that causes sickle cell disease provides a protective advantage against malaria, particularly in regions where the disease is endemic. Individuals who are carriers (heterozygous) for the sickle cell trait have some resistance to malaria, which can be life-saving. However, when a person inherits two copies of the sickle cell allele (homozygous), they develop sickle cell disease, leading to severe health complications, such as pain crises, organ damage, and reduced life expectancy. Thus, the allele's dual role illustrates a classic example of balanced polymorphism in human genetics.