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 circumstances, it confers an characteristic under normal advantage in these areas. Hence, there is a high prevalence of this trait. Although the reason is still unknown, researchers believe the presence of the sickle-cell allele stimulates the immune system, providing some protection against malaria