What if invisibility is the function of a parasite?
Invisibility is caused by a symbiotic organism which interacts with photons in such a way that renders the host invisible. Maybe it absorbs some energy from the light, like wavelengths invisible to humans? Perhaps the relationship is heat-related: an exothermic symbiote needs the body heat of an endothermic host? Or even more fundamentally, perhaps it's a virus, depending primarily on the metabolism of the host for its survival?
How does this work? What does it mean?
What is the story?