Knowing the way children and adolescents assess the risk of disease transmission is important because this kind of knowledge may allow health caregivers to better communicate with them. We had 587 students in Spain and France aged 7-16 judge the risk of disease transmission in 28 scenarios of students visiting a sick friend. The scenarios were composed according to a three within-subject orthogonal design: type of contact (from no contact to prolonged close contact), type of disease (contagious vs. noncontagious), and number of contacts. Cluster analysis revealed six main clusters, labeled "Contagion can never been ruled out," "Depends on type of physical contact," "Depends on type of disease," "Depends simultaneously on contact and disease," "Depends conjunctively on contact and disease," and "Categorical thinking." The composition of these clusters depended strongly on age level. From a practical perspective, even 7-year-olds seem to be fully aware of the issue of contagion, although they overgeneralize the risk. As a result, progress in understanding might be best achieved through a process of unlearning rather than learning. Highlights: We examined the way children and adolescents judge the risk of contagion.Five hundred eighty-seven students aged 7-16 judged the risk of disease transmission in 28 scenarios of students visiting a sick friend that were composed according to a three within-subject design: type of contact × type of disease × number of contacts. Six judgment rules were found. They were labeled "Contagion can never been ruled out," "Depends on type of contact," "Depends on type of disease," "Depends simultaneously on contact and disease," "Depends conjunctively on contact and disease," and "Categorical thinking." Even 7-year-olds seemed to be fully aware of the issue of contagion, although they overgeneralize the risk. As a result, progress in understanding might be best achieved through a process of unlearning rather than learning.