CHAIN's initial aim is to better understand the characteristics that determine increased risk of mortality in this vulnerable population, whether biological (related to infection, immunity and metabolism), nutritional (intake and anthropometry), health system factors (affecting management and discharge) or behavioural (community/caregiver interactions). CHAIN brings clinicians and scientists together from a variety of high-burden settings in Africa and South Asia. CHAIN will establish prospective cohorts of acutely ill young children across different geographies with differing population, social and environmental characteristics (stunted vs. wasting, rural vs. urban and presence of risk factors such as malaria and HIV).