Based on the analysis of the Medical and Public Health Research programme (Commission of the European Communities), the paper shows how new scientific communities are created in response to the HIV/AIDS problem. We analyze how actors are mobilized (three mobilization modes : public impetus, scientists'initiative, scientists' initiative with public networking) and how their work is coordinated. We defend the hypothesis that these new scientific communities are flexible cooperation networks. In the case of AIDS research, there are only a limited number of network types (the data collection structure, the forum, the thematic partition with harmonization of research practices, the starred around a central facility). The coordination of these scientific cooperative networks passes through fixed and circulating intermediaries. The management of these intermediaries is a major activity for involved actors. thus, our aim is not to study the wider social context, but to analyze networking in response to policy initiatives. (Résumé d'auteur)