This article analyzes the path that led the European Union from a somewhat accidental involvement in fighting AIDS to a new and sustainable policy of communicable disease control. It responds to three main questions: Why did an unexpected case lead to the organization of a new sustainable policy? How was this achieved despite national competency over the given policy sector? How did the new policy succeed in covering the enlarged EU? The explanation combines political factors and public health issues. European integration and eastern enlargement made transborder disease management a political necessity. Treaties gave legitimacy to EU policy, while the AIDS matrix furnished the practical procedures: networking, data harmonization, peer-conducted policy coaching, and participation. This pattern of public health management is compatible with national competency, and it is consistent with the democratic values the EU promotes throughout and beyond the enlarged EU. From a theoretical point of view, these patterns of activities constitute the empirically grounded content of the much used but less defined concept of cognitive Europeanization.