Abstract: A method of controlling the spread of an infectious microorganism, for example methicillin-resistant Staphylococcus aureus (“MRSA”) in a group of subjects including people asymptomatic for infection with the microorganism. A dose of microorganism-reducing light can be applied to each anterior nasal cavity of each subject in the group. Optionally a colorant can be applied to the anterior nasal cavity to sensitize any infectious microorganisms present to the microorganism-reducing light. A light applicator system having a hollow light-transmissive nasal dilator insertable into a nostril and a light output member accommodatable in the hollow interior of the nasal dilator can be employed, optionally with a light diffuser around the light output member. One example of light applicator system comprises a hand piece removably attachable to an optical fiber coupled to a light source, the optical fiber constituting the light member.