Arrangement for reducing micro-organisms comprises a radiation device with a light source (26), a photo-sensitive substance which treats the area to be treated and is irradiated by the light source, and an applicator containing a fiber-optic waveguide (2). The applicator and radiation device comprise connecting bodies (4, 50) contacting one of them in such a way that the light from the light source is emitted towards the treated area using the waveguide.
