Guided by both a Piagetian and a naïve theories perspective on disease concepts, this study examined children’s thinking about HIV/AIDS, with special attention to its development, coherence, and sociocultural correlates. It examined age differences among Mexican-American and Euro-American children aged 8 to 13 (N = 158) in both Piagetian level of causal understanding (independent of correctness) and the causal knowledge central to an intuitive theory of AIDS (knowledge of risk behaviors and of the viral disease agent). It explored theoretical coherence in terms of implications of causal understanding and causal knowledge for knowledge of how to prevent AIDS and willingness to interact with people who have it. As predicted, scores on all measures increased significantly with age, and causal knowledge of risk factors exceeded knowledge of corresponding prevention rules. In multiple regression analyses, causal knowledge of both risk factors and the viral disease agent predicted knowledge of prevention and willingness to interact, even with age and other measures controlled. Prevention knowledge predicted willingness to interact even better, whereas the Piagetian measure of casual understanding did not predict either prevention knowledge or willingness to interact. Ethnic group differences were not evident but parent education was related to greater viral knowledge and willingness to interact. The results suggest a good deal of coherence in children’s thinking about this disease while also suggesting the desirability of making explicit the implications of critical causal information about an unfamiliar disease for preventing the disease without stigmatizing those who have it.