Starting from the realizing that prevention of Aids among adolescents and youths is important whereas the epidemics is of mediocre concern to that population, the author begins to ponder on the complexities of the social representations associating Aids and adolescence, starting from the idea that such categories result from a task of the social institutions. Hence developing a prevention discourse on Aids with those categories entails Aids to be thus integrated into a consensus discourse rather that speaking in terms of marginal categories (homosexuals, addicts, African migrants). When referring to both these categories, what is at stake are patterns of social functioning (the biographical institution in the case of adolescence, the territorial group for youth). Such a reading of the action of prevention shows the underlying political splitting, i.e. on the one hand, adolescence that should be protected from the risks run by Aids ; on the other hand, youth that should be protected from the risk run by society due to Aids.