THE α⁺-thalassaemias are the commonest known human genetic disorders, affecting up to 80 per cent of some populations¹. Although there is good evidence from both epidemiological^2,3 and clinical⁴ studies that these gene frequencies reflect selection by, and protection from, malaria, the mechanism is unknown. We have studied the epidemiology of malaria in childhood on the southwestern Pacific island of Espiritu Santo in Vanuatu and here we report that, paradoxically, both the incidence of uncomplicated malaria and the prevalence of splenomegaly, an index of malaria infection, are significantly higher in young children with α⁺-thalassaemia than in normal children. Furthermore, this effect is most marked in the youngest children and for the non-lethal parasite Plasmodium vivax. The α⁺-thalassaemias may have been selected for their ability beneficially to increase susceptibility to P. vivax, which, by acting as a natural vaccine in this community, induces limited cross-species protection against subsequent severe P. falciparum malaria.