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Malaria has been prevalent in many tropical and subtropical regions of the world — regions that are native to the peoples in which sickle cell anemia disease is prevalent. This observation gave rise to the establishment of a connection between sickle cell anemia and malaria. Researchers have also affirmed that people with sickle cell trait, which is not the actual disease, present a greater resistance to developing malaria than people without the disease or the trait, thus establishing another connection between these two diseases. Some scientist also believe that this information proves the work of the theory of natural selection. They conclude that evolution connects sickle cell anemia and malaria in a positive light by suggesting that the body's ability to develop deformed or sickle-shaped red blood cells made native populations of the tropics "fit" to survive.
While it is not known exactly how people with the sickle cell trait are better able to resist malaria, researchers believe that a number of factors are involved. For example, the red blood cells of a person with the trait might sickle when oxygen tension is low in venous blood, which is the blood in the veins. It is suspected that infection of the red blood cells with malaria-causing parasites also causes low oxygen tension and leads to sickling. Sickle-shaped blood cells might be viewed by the immune system as invaders and destroyed before the parasites have a chance to completely invade. Some also suggest that parasites responsible for malaria might be destroyed directly within the sickle trait cells themselves.
Some people, however, have disputed the connections between sickle cell anemia and malaria. The fact that a person with sickle cell trait generally has a genetic resistance to malaria does not at all mean that he or she is immune to the parasite responsible for the disease. Such an individual has a 25 percent or higher chance of succumbing to malaria and dying from it. This is one of many facts that cause some medical doctors, researchers, scientists and laypeople to reject the natural selection theory linking the two diseases. Natural selection would have failed to consider worldwide migration of populations native to the tropics and subtropics to regions such as Europe and North America, where malaria has never been as prevalent as it once was in warm, humid climates.
Sufferers of sickle cell disease usually end up dying prematurely from the development of deformed red blood cells introduced through the supposed protections of evolution. Another consideration that causes dispute regarding a natural and beneficial connection between sickle cell anemia and malaria is the fact that native foods rich in a substance called thiocyanate, such as cassava and African yam, were staples in Africa. Thiocyanate has been proven to be an anti-sickling agent, and during the times when these foods were eaten as staples, sickle cell disease was virtually unknown. If sickle-shaped blood cells are beneficial in helping to guard against malaria, the question of why foods with natural anti-sickling properties would be staples in a malaria region has caused some people to doubt a natural link that is beneficial between the two sicknesses.