The purpose of this study is to find out how, why, and when Rift Valley Fever (RVF) spreads. Participants will be 250 adults and children, aged 1 year and older, from the Ijara District, Kenya. They will be given a questionnaire, undergo a medical examination that includes an eye exam, and have a 1-teaspoon sample of blood taken from a vein. Participation will take about 3 hours.