Five experiments demonstrate how potential moral stigma leads people to underplay their susceptibility to sexually transmitted diseases (STDs) and dampens their interest in getting tested. After adding unprotected sex to a list of otherwise innocuous possible vectors for a disease, the authors found that infected people were perceived to be less moral (Experiment 1a), and individuals believed that if they had the disease, others would see them as less moral too (Experiment 1b). Adding this stigmatized vector also reduced reported testing intentions (Experiment 2) and perceived risk of exposure (Experiment 3)--a disjunction fallacy because adding a potential cause reduced estimated likelihood, in violation of basic probability rules. Finally, the authors replicated the effect in a computer virus analog (Experiment 4) and showed that it did not result from simply knowing that one has not engaged in the stigmatized behavior. Results suggest that avoidance of potential stigma can have dramatic health consequences, both for an individual's health decision and for health policy.