AIDS is a difficult health issue which mainly concerns medicine, hospitals and public health policies. But the epidemic also concerns the political system. The definition of the nature of risk and of the rank it should occupy amongst the different risks society may be facing is a political task. A new problem and its possible consequences can be evaluated in many different ways. Different social groups will develop various conceptions of the problem according to their position in society and of the danger it may present for them and for society as a whole. Therefore, negotiation is needed between different possible perceptions of the problem and a large consensus is necessary to make the choice legitimate. Priorities as defined will automatically legitimate certain actors and certain means to deal with the problem and exclude others which are considered as irrelevant. The AIDS epidemic offers an excellent case for studying these strategic moments of problem definition and consensus building which shape public policies from their very beginning.

International comparison is of particular interest here, because every country faces a similar problem. But as no legitimate knowledge pre-existed to interpret the new epidemic when it occurred and the threat it might represent to society, problem definition and policy construction largely depended on the characteristics of national policy systems, institutional networks and decision making processes in health policies.