In regions where malaria is prevalent, the s allele is beneficial because: it causes red blood cells to distort, which makes them less hospitable to plasmodium, the parasite that causes malaria. it prevents mosquitoes from releasing plasmodium, the mosquito that causes malaria, into the blood. it improves the ability of the hemoglobin to carry oxygen. it improves the ability of the immune system to fight plasmodium, the parasite that causes malaria.