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?


3 Answers 3


Let's flip-flop @Henry Taylor's answer: the diseases flourish with nourishment, which they can only find underground.

Just like you can't uproot a tree and place it in a parking lot (because it needs water, soil etc), your diseases may require local conditions that the surface does not have.

  • Rocks or dust from them (salt, quartz, clay, silicates)
  • Atmospheric conditions (proportions of gas or lack of oxygen)
  • Lack of light in general (some fungi may grow best in the dark)
  • Moisture (or lack thereof, depending on the local climate)
  • $\begingroup$ Accepting this, as being sensitive to moisture would work very well (the "infected" region is near the coast and gets a lot of rain) $\endgroup$
    – user7076
    Commented Feb 24, 2017 at 17:39

Rather than killing the virus when it reaches the surface, why not follow scurvy's example and make it easily treatable by proper nutrition. Perhaps the underground dwelling human eat only fungus and rats while the surface dwellers get a balanced diet of fruits, vegetables and meats. The diseases express themselves dramatically in the malnourished bodies of the orcs, while remaining completely suppressed in the well fed non-orcs.


Related to Henry Taylor's answer but without the need for malnourishment:

In addition to UV sensitivity the disease is sensitive to vitamin D levels. The surface dwellers make plenty of vitamin D and the disease can't take hold. Those who have to rely on dietary D, however, have lower levels and the disease hits them.


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