Abstract: The present invention discloses a selective and a non-selective method and means of treating microbial diseases in local infections. The novel method comprising the step of emitting a beam of a blue light towards the tissue to be treated, wherein the beam is having a wavelength from 400 to 550 nanometers and further wherein the temperature of said tissue is not exceeding about 42° C. along the procedure or immediately after it so that a selective phototoxic effect is exerted. A cost effective device for selectively or non-selectively exerting phototoxic effect by emitting a beam of blue light towards a tissue to be treated is also provided useful, especially for dental application.