In many societies around the world, people attribute illness to supernatural forces, including deities, spirits, and malevolent agents. Using observational data from sub-Saharan Africa and an original large-scale survey in the Democratic Republic of Congo (DRC), I document the near-universality of supernatural beliefs about illness -- 94\% of respondents hold at least one such belief -- and their relevance for health behavior: They are linked to lower use of and beliefs in the effectiveness of modern medicine, and higher stigma toward those with illness. Then, I conduct a field experiment in the DRC to test whether these beliefs can change. I randomize showing an informational video about the biomedical cause and treatment of epilepsy, a prevalent disease commonly associated with supernatural forces. The intervention shifts respondents' beliefs away from supernatural causes and toward modern medicine's effectiveness, not only for epilepsy but for other conditions. Moreover, the intervention reduces stigma toward those with the disease and increases take-up of free hospital consultations for epilepsy by 50%.