Devices and methods for impinging light on tissue to induce one or more biological effects, and more particularly illumination devices and related methods that may be used for intranasal delivery of irradiation are disclosed. Exemplary illumination devices may include a light guide that is optically coupled with a light source, where the light guide may be configured for insertion along one or more intranasal passageways. In this manner, the light guide may provide irradiation of light to tissues along or near the upper respiratory tract to prevent and/or treat various infections and other tissue conditions thereof. Light guides may include flexible materials with suitable dimensions and/or shapes that allow the light guides to follow variable paths of intranasal passageways during use.