A chronic, severe, and sometimes fatal anemia marked by crescent-shaped red blood cells and characterized by fever, leg ulcers, jaundice, and episodic pain in the joints. The disease occurs in people who are homozygous for a gene that produces an abnormal form of hemoglobin, and it is found chiefly in people of African descent and in some Mediterranean, Middle Eastern, Central and South American, and South Asian populations. Also called sickle cell disease.