In societies where formal channels of diffusion of health risk and prevention-related information and practices are inadequate, informal social networks offer a powerful alternative. This study compares how social networks deal with two types of public health concerns - the prevention of cholera and HIV/AIDS. It is based on data from in-depth interviews and focus groups conducted in Maputo, Mozambique, in June-August of 1998 - in the aftermath of a major cholera epidemic and in the midst of an unraveling catastrophe of HIV/AIDS. For both diseases interaction through social networks compensates for the insufficient and inadequate information distributed through official channels. However, the differences in the latency period, course, and clinical manifestations of the two diseases affect the nature, mechanisms, and forms of social interaction. Because HIV/AIDS remains largely an abstract threat not supported by practical knowledge and everyday experience, HIV/AIDSrelated social interaction transmits mainly rumors and unconfirmed suspicions. In addition, this interaction is constrained by traditional social barriers and distances, such as the ones based on gender and age, which may retard the rise of HIV/AIDS awareness in the community. In contrast, the symptoms of cholera are easily identified and understood, and the generalized threat of the epidemic spawns intensive and widespread interaction both within and across conventional social boundaries. This interaction mobilizes the community against the epidemic and helps individuals to improve the prevention of infection.