Well any planet anywhere can signify physical functions, or more precisely problems with physical functions. Mercury traditionally represents the brain, memory, voice, speech, the organs of speech and according to the seventeenth century medical Astologer, Richard Saunders, the sinews (muscles) hands, feet and little finger. Modern Astrologers would probably add the nervous system and respiratory system.
All of these are related to Mercury functions - communication, by speech or gesture, thought and mobility. So in a decumbiture chart (for an illness), if Mercury ruled the sixth house of dis-ease, then the problem lies in any of the above. Now Mercury in Pisces is in both detriment and fall. So if Mercury rules the sixth but is postioned in Pisces, any of the Mercury physical functions would be seriously debilitated. Paradoxx's migraine or purplefishy's pinched muscle would clearly fit this description of Mercury ailments.
Is it likely to affect you?
Well if you have a Gemini or Virgo Ascendant, and Mercury is in Pisces then yes, you do carry the risk - more Gemini than Virgo (Virgo's big problem is with the digestive system). If you have Mercury in Pisces in the sixth, then again this might be your most likely dis-ease, though you should have a look at the ruler of the Ascendant first.
We rarely consult Astrologers these days about illness but a chart cast for the moment that you were ill enough to take to bed, might reveal Mercury ruling the sixth and in Pisces, or Mercury in Pisces in the sixth. In the latter case I'd tend to go for foot related problems such as Achilles tendon, muscles controlling the foot movement, or nerve problems which affect walking as Pisces also relates to the feet.
