Why is sickle cell disease most common among people living in tropical regions, and their descendants?

Sickle cell disease is caused by a recessive allele of the haemoglobin gene. When two copies are present (i.e. it is homozygous), many red blood cells become sickle-shaped, leading to disease.

However, if only one copy of this allele is present (heterozygous, or "sickle cell trait"), the carrier does not display symptoms. Additionally, the single copy confers resistance to malaria - a disease most common in tropical regions (e.g. sub-Saharan Africa). So in these regions, people with sickle cell trait are more likely to survive and reproduce than those without. This amounts to a selective advantage for the sickle cell allele.