How could an airborne disease only survive undeground?

The orcs in my world are humans who have been infected by two different diseases:

• An airborne mutagen that permanently denatures their myostatin receptors, which prevents their muscles from being growth-limited like a normal human. This causes increased muscle mass, though not without issues.
• A greenish skin fungus. This visually resembles psoriasis, but is completely harmless to humans (unlike e.g. rats).

I want both of these to only appear underground, and never on the surface. My first thought is that ultraviolet light would do the trick, but that requires direct sunlight. For the skin fungus, that isn't a problem (since ultraviolet light can be used to treat some skin conditions), but this leaves the mutagen. How can I make it die when it gets close to the surface?

posted almost 8 years ago

Philip Rowlands‭

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