This study specifically seeks to document infection by Plasmodium species parasites in a prospective longitudinal study using blood smear and polymerase chain reaction (PCR)/Ligase Detection Reaction (LDR) diagnostic techniques and to determine when a child becomes susceptible to blood-stage malaria in a cohort of non-immune children from villages where a genetic polymorphism that blocks expression of Duffy (Fy) blood group antigen has been identified. The goal of this project is to prospectively study the natural history of Plasmodium vivax malaria in a malaria-endemic region of Papua New Guinea among a cohort of children 24 months of age and older for a period of up to two years. The primary objectives of this study are to: 1) determine the mean time to first P. vivax infection, and 2) determine the incidence rate of P. vivax infection. The secondary objectives of this study are to: 1) assess the impact of recurrent Plasmodium infections on child growth, 2) estimate the sensitivity and specificity of blood smear versus PCR-based diagnostic methods for P. vivax infection, and 3) evaluate the effect of the FY genotype on P. vivax malaria. Cross-sectional malaria prevalence data based on blood smear diagnosis suggest that the risk of malaria is extremely low during infancy in this population. Therefore, at the start of the study, children 12 months of age and younger will be eligible for the study, and all new births occurring over the next 15 months will be added to the cohort ensuring that the last newborn will have a minimum of 9 months of follow-up. All children enrolled in this study will be followed weekly to detect clinical malaria and biweekly blood samples will be taken for the detection of asymptomatic malaria. Follow-up will continue for a period of up to 2 years until participants reach 24 months of age, or withdraw from the study or until the study ends, whichever event comes first.