We present a simple political economy model that explains two major puzzles of government policies to combat HIV/AIDS epidemic: the lack of policy response in many countries where the epidemic is massive and the reversal of the downward trend in HIV prevalence in the countries that have adopted early agressive prevention campaigns. The model builds on the assumption that the unaware citizens impose a negative externality on the aware by increasing the risk of contagion. Prevention campaigns raise awareness of the current generation, which then partially transmit this awareness to the next generation, thus creating political support for the next-period awareness campaigns. The economy has two steady-state equilibria: the "good" one (with high awareness and low prevalence) and the "bad" one (low awareness, high prevalence). The "good" equilibrium is fragile, i.e. a sufficiently large exogenous drop in HIV prevalence undermines the next-generation political support for campaigns and makes the economy drift away towards the "bad" equilibrium.
