Despite the well-established literature on explanation in early childhood, little is known about what constrains children's explanations. State change and negative outcomes were examined as potential explanatory biases in the domain of naïve biology, extending upon previous work in the domain of naïve physics. In two studies, preschool children (N = 70, 3- to 5-year-olds) were informed of the distinct health outcomes of characters in four between-subjects conditions (i.e., becoming ill, recovering from illness, continuous health, and continuous illness) and were asked to provide explanations. Whereas children in both studies provided relevant information for health outcomes, they more often explained outcomes that included a salient health-state change. Presence of a state change also influenced the interpretation of potentially relevant information and improved memory for health outcomes. We discuss how biases in children's explanations constrain children's reasoning and may exacerbate difficulties with reasoning about important health-related topics such as illness prevention.