Why sickle cell anemia and malaria found in same geographical area?

It's called selective advantage. Malarial parasites reproduce within red blood cells. Plasmodium falciparum, one of the parasites that causes malaria can not reproduce inside the defective sickled cell. Therefore, the people with sickle cell trait (one gene defect) are less susceptible to the parasite. However, those with two gene defect, or one from each parent are usually more sick with their sickle cell disease. Malaria is endemic in Africa and this is where the majority of people with sickle cell anemia trace their origins.