Guided by a naïve theories perspective on the development of thinking about disease, this study of 188 children aged 6 to 18 examined knowledge of HIV/AIDS causality and prevention using parallel measures derived from open-ended and structured interviews. Knowledge of both risk factors and prevention rules, as well as conceptual understanding of AIDS causality, increased with age. Younger children displayed more advanced knowledge in response to structured questions than in response to open-ended questions. Contrary to hypothesis, knowledge of causality was not more advanced than knowledge of prevention in elementary school. Moreover, correlations between the two types of knowledge were often nonsignificant except when the same method was used to assess both. Thus, methodology matters in assessing children's knowledge of disease, children's intuitive thinking is not consistently coherent, and it may be safest to educate children explicitly about sound prevention rules rather than assume they will infer the rules themselves from information about a disease's causes.