We study how social proximity between the sender and the receiver of information shapes the effectiveness of preventive health behaviour campaigns and the persistence of misinformation. We implement a field experiment among a representative sample of slum residents in two major Indian cities characterized by Hindu-Muslim tensions. We show that informative messages are effective at improving evidence-based behavior, but not non-evidence-based behavior. These findings do not differ by social proximity, signalled by religion. However, when sender and receiver share the same religion, the intervention significantly reduces misinformation carrying in-group salience, highlighting the role of social proximity in fighting misinformation.