In developing countries HIV and AIDS are widespread, especially in sub-Saharan Africa. Most authors attribute the spread of the disease to economic and social factors such as poverty, unequal sexual relationships and lack of condoms. This article tries to show that these social factors are not sufficient to explain why people do not modify their risky sexual behavior in societies with infection rates up to 40% but behave in the same way as they did before AIDS was known. An analysis of cognitive structures may help to understand why a disease, transmission of which is easy to avoid by behavior change, could have disseminated so much. One important cause is that people do not recognize HIV/AIDS as a natural and physical phenomenon that can be treated and encountered by cautious and careful behavior. It is seen as a magical power that can only be fought by mystical forces, not by scrupulousness in sexual behavior and relationships. Preoperational modes of thought (Piaget, 2001) can explain this kind of world view and behavior.