Public health data highlight disease outcome disparities corresponding to age, social class, and race, which are due to biological, behavioral, and/or structural factors (depending on the disparity). We examined whether adults are aware of these disparities and how they explain them. This study recruited U.S. adults (N = 241) through Mechanical Turk and examined whether they thought that there was a relation between social categories and illness. We examined their judgments and explanations for transmitting and contracting COVID-19 or the common cold. We found that adults thought that older adults and poor people were more likely than younger adults and rich people to get sick, whereas younger adults were more likely than older adults to transmit disease. People relied on biological explanations for disparities due to age, and structural explanations for disparities due to social class. However, the results for race were more mixed, suggesting that people do not always assume that social categories are related to illness.