HIV prevalence in Uganda has leveled off, however trends indicate that incidence is on the rise and disproportionately affects certain vulnerable groups, such as women. There is growing support for using mobile health (mHealth) programs to reach vulnerable populations. Using the Extended Technology-Community-Management model for mHealth, we examined a text message campaign in Arua, Uganda, that disseminated and measured HIV/AIDS knowledge and promoted HIV/AIDS testing at clinics. Empirical data from this campaign illustrated that mHealth campaigns need to address socio-cultural, informational, economic and individual vulnerabilities. We found, for example, that the interactive SMS quiz design motivated recipients with the correct HIV/AIDS knowledge to respond (and thus become eligible for free HIV screening). In doing so, however, the more vulnerable sections of the population, such as those with incorrect knowledge, were further put at risk. We discuss the programmatic implications and provide strategic recommendations for campaign focus, particularly to reach young women.