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The evolutionary process of natural selection ensures that when a disease afflicts a group of people, those who do not have the genes that provide them protection against the disease die; and the ones left alive pass on to their offspring genetic traits which are different and which allow their bodies to protect them from the disease.
Some examples of these genetic defenses include the presence of the sickle-cell gene in African blacks that protects them from malaria, the Tay-Sachs gene in Ashkenazi Jews which protects them from tuberculosis and the cystic fibrosis gene which protects northern Europeans from bacterial diarrhoea. (Diamond, Jared, 1997, Guns, Germs, and Steel: The Fates of Human Societies, p. 201)
sickle cell because people with it are immune to malaria :D
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