The purpose of this study is to learn more about vivax malaria in very young children, how the growth of children is affected by malaria infections, and how inherited traits protect children from getting malaria. Participants will be 266 children 24 months of age or younger who live in certain villages in East Sepik Province, Papua New Guinea, where malaria is very common. The study also will find out how malaria spreads in the area. By learning how and when a child develops resistance to vivax malaria, researchers can try to find ways to help people from getting it. And, they can get a better understanding of how inherited traits influence all types of malaria infections. In this study, the parent and child will be visited by the study team or health workers every week to check health and bednet use. Every 2 weeks, the study team will take a small blood sample to check for malaria and take measurements to study the child's growth. Participants will be followed for up to 2 years.