one example is the prevalence of the sickle-cell allele in malaria-infested regions of africa. Although the sickle-cell trait or anaemia is not an advantageous charecteristic under normal circumstances, it confers an advantage in these areas. Hence, there is a high prevalence of this trait. Although the reason is still unknow, researchers believe the presence of the sickle-cell allele stimulates the immune system,providing some protection against malaria