A system and method for light stimulation within a medium. Products, compositions, systems, and methods for modifying a target structure which mediates or is associated with a biological activity, including treatment of conditions, disorders, or diseases mediated by or associated with a target structure, such as a virus, cell, subcellular structure or extracellular structure. The methods may be performed in situ in a non-invasive manner by application of an initiation energy to a subject thus producing an effect on or change to the target structure directly or via a modulation agent. The methods may further be performed by application of an initiation energy to a subject in situ to activate a pharmaceutical agent directly or via an energy modulation agent, thus producing an effect on or change to the target structure. Kits containing products or compositions formulated or configured and systems for use in practicing these methods.
