Individuals with the Duffy negative blood group are highly resistant to blood stage infection with the malaria parasite Plasmodium vivax. Duffy negativity is so widespread in sub Saharan Africa that transmission of P. vivax malaria cannot be maintained in this region. But why has Duffy negativity not reached similar frequencies in other parts of the world where P. vivax is endemic? Are malaria blocking mutations, or, more broadly, infection blocking mutations, likely or unlikely to reach high frequencies in populations? In this talk I will present a theoretical framework with which to explore these questions, and demonstrate a possible explanation for the global distribution of the Duffy blood group.