The notion that mass screening and treatment of HIV could effectively end the epidemic has generated enthusiasm in global health. The strategy of “treatment as prevention” (TasP) is being experimented with in population trials even as it is being adopted by physicians and, increasingly, policymakers. This chapter explores the genesis of TasP and suggests that the modes of implementation of this approach may have unintended and paradoxical effects in settings where social services are more likely to be delivered by ngos, humanitarian interventions, and research institutes than by the state. Large prevention experiments, such as TasP, have the potential to produce biological and social effects beyond the actual experiment that could be qualified as a para-state.