In tropical countries, concomitant infections are a continuous problem. In the Rufiji Delta, an area of Tanzania that is holoendemic for malaria, there were outbreaks of influenza A, measles, and pertussis in 1986 and 1987. Significantly lower parasitic prevalences and mean densities of malaria parasites were found in children up to nine years of age who had measles or influenza than in asymptomatic control children. In contrast, children with pertussis had a higher prevalence and mean density than controls. The clinical courses of measles, influenza, or pertussis infections did not appear to be significantly affected by concomitant malaria infections. The reasons for the suppression of Plasmodium falciparum parasitemia during these viral infections are unclear. This effect could not be explained by the presence of fever.