The purpose of this study is to see why malaria epidemics occur in highland areas in Kenya. A better understanding of factors contributing to malaria may be necessary for malaria vaccine planning. These factors include interactions between age, where malaria is passed from mosquitoes to people, immune system (how the body fights infection) responses and other factors that contribute to malaria in epidemic-prone areas. About 6400 people from the villages of Kapsisiywa and Kipsamoite will participate. Study procedures will include in home surveys, which will involve a census and an interview by researchers. Blood samples and smears will be collected from some volunteers in both communities to understand how the body protects itself from malaria and to check for malaria parasites. Twice each month, random houses will be selected from 3 places in the village to measure the number of mosquitoes in the home. Participants may be involved in the study for up to 4 years.