Humans and other living organisms harbor disease-causing pathogenic microorganisms. These microorganisms are often transmitted through physical contact with contaminated objects, such as food, water, or other people. While some theoretical and empirical research examines the ontogeny of contamination and contagion beliefs, cross-cultural research on this topic is limited. To help remedy this paucity of data, we conducted an ethnobiological study of contagion and contamination beliefs among Maasai children (n = 42) in the Simanjiro district, Tanzania. Participants include 36 middle-aged schoolchildren and six four-year-olds. We contrast the children's beliefs with those of 12 local adults. To measure children's views of contagion and contamination, we developed sentence-framed yes/no elicitation tasks using three different stimuli—a fly, a cough, and a cough from someone with respiratory symptoms. Qualitative semi-structured interviews were conducted to further understand children's ethnotheories of contamination and contagion. Children generally reported that coughs and flies are directly contaminating, whereas they offered mixed results for associational and indirect contamination. Children discussed time, psychological contagion, saliva, wind, and the supernatural/natural as key elements to their beliefs, reasons, and personal actions taken to minimize contamination/contagion risk. We found education to be significantly positively correlated with children reporting that flies and coughs were directly contaminating, while age had no effect. Although children were more likely than adults to associate flies with contamination, their general belief pattern differs little from adults. Local cultural-ecological factors and explanatory models of disease, as well as formal religious and education institutions, shape Maasai children's ethnobiological beliefs of contagions and contamination.