Research on the development of contamination sensitivity, detecting when water or food is safe to consume, has been conducted mainly in western, industrialized settings. Little is known about it in the developing world where contamination is a significant health hazard. In this study, 48 children and adults (4 to 60 years of age) in rural Uganda were asked if 23 food and water items, shown in photographs or drawings and that varied in contamination status, would be safe to eat or drink. They were also interviewed about several of the photographs in more detail and asked how they learned about contamination. Analyses focused on age-group differences. Contamination knowledge, biological explanations of illness, and awareness of invisible contaminants (i.e., germs) were evident in all age-groups. The few age-related differences found pertained to water contamination and some vulnerabilities of young children (4–6-year-olds) were identified. Participants in all age-groups said they had been taught to avoid contaminated substances and how to handle food and water safely. Implications for health interventions in rural Uganda and similar areas of the developing world are discussed.