The invention relates to a treatment device which is intended for a filter, such as a particle filter (1) or a combined filter of a heating, ventilation and/or air-conditioning system for a vehicle passenger compartment, in order to prevent the development of micro-organisms on the surfaces of one such system. The device essentially consists of a permeable container (6) containing a volatile treatment agent and comprises means for fixing the container (6) to the filter between the folds in the filtering structure (3) thereof, such that the container (6) can be installed on any filter (1) of an existing system in order to prevent the development of micro-organisms on the surfaces of said system.