Africans may become susceptible to a type of malaria that they had long been thought to be immune to because the parasite appears to be evolving, scientists said last week.

Researchers working in Madagascar, an island off the African coast, found that 10 percent of the people they tested who had vivax malaria were “Duffy-negative,” meaning they lacked the Duffy proteins on the surfaces of their red blood cells to which vivax malaria parasites attach.

Duffy-negative people have been thought to be nearly immune to vivax malaria; 95 percent of all Africans are Duffy-negative, so vivax malaria is rare on that continent. In Asia it is widespread.

Falciparum malaria, caused by a different parasite that is not affected by Duffy proteins, kills about 650,000 people a year, mostly in Africa. Vivax malaria is not as lethal, but is harder to cure; victims can have repeated relapses.