CYP2D6 and Plasmodium falciparum malaria: Human polymorphisms known to reduce the susceptibility to P. vivax infection (e.g., Duffy blood group negativity) or affect the efficacy of antirelapse treatment (e.g., low-activity CYP2D6 variants) have no such effects on falciparum malaria risk; these are “vivax-specific” malaria resistance factors.