Sickle-cell disease is a debilitating disease that results from being homozygous recessive for the "sickle-cell allele" of a hemoglobin gene. Why is the allele present and even common in many human populations?

Heterozygote advantage

Explanation:

The sickle cell allele in a heterozygote state confers a kind of resistance to malaria in areas where the disease is endemic while it is deleterious in homozygous condition. Hence, this heterozygote advantage ensures that the recessive allele is present and quite common in the population.