In 1993, the parasite cryptosporidium infested the Milwaukee-area drinking supply and sickened some 400,000 people. This study uses survey data gathered from 610 residents in the wake of that outbreak to look at predictors of the complexity of people’s understanding of two causal components of the outbreak: (1) how the parasite got into the water and (2) how it caused illness in the human body. Analysis of open-ended data indicated that, consistent with the predictors used in the knowledge gap literature, indicators of socioeconomic status are significant predictors of differences in explanatory complexity. Also, consistent with the literature on motivation and knowledge seeking, experience with and worry about the parasite served as predictors of explanatory complexity under certain circumstances.