Detailed data on morbidity is collected through means of a prospective follow up on a sub-sample of 420 children from 6 sentinel villages (210 from group A and 210 from group B). These 420 children are visited daily by field workers who measure their temperature. In case of fever, field workers take a blood sample through finger prick to be analysed for malaria parasitaemia. Treatment free of charge is organised for all children in this subsample. In addition, these children are visited twice a year for the collection of clinical (malaria episodes, anaemia) and parasitological (rates of malaria parasitaemia, parasite density) parameters. Data collection on this subsample of children is meant to last from June 2000 to December 2003. For study purposes, falciparum malaria is defined as 37.5 C or more plus at least 5 000 parasites per micro-litre.