Studies undertaken in Guinea Bissau have shown that the protective effect of BCG vaccine against death in young children is greater than would be expected from its effect in preventing tuberculosis. How this might be achieved is not known but BCG could act by enhancing the immune response to common infections including malaria which is an important cause of death in young children in Bissau. There is some laboratory evidence to support this hypothesis. BCG prevents malaria in rodents, probably by activating their reticulo-endothelial system and Freund's adjuvant, an emulsion of tubercle bacilli in oil, prevents malaria in primates.