Nomadic Turkana pastoralists of northwest Kenya have been studied for a number of years as part of a multidisciplinary project concerning the people and their ecosystem. Among the most interesting components of the human studies is the demography of the Turkana; although they inhabit an extremely harsh environment sufficient to impact health and reproduction, the Turkana experience an unusually high fertility level for nomadic populations, with a very marked seasonal distribution of births. Recent studies, finding extremely high rates of infant and childhood mortality, suggest that the fertility pattern may largely be explained by the pattern and level of mortality. The research proposed here is designed to examine the pattern of morbidity and mortality among infants and children, and to identify determinants of observed patterns. Traditional demographic studies of the causes of morbidity and mortality have emphasized correlations among socioeconomic factors, such as household size, and risk of disease and death, and have largely ignored underlying biological processes. This study will employ recently developed immunological techniques to measure precisely the effect of nutrition and infection on human health and survival, and will link these factors to analyses of sociocultural predictors of risk of disease and death. The results of this work will contribute to our understanding of micro-level causes of morbidity and mortality, and will facilitate the development of a unified biological and demographic approach to the study of child survival.