Some inspiration from : Pityriasis Rubra Pilaris
Shimerscale is a disease that affects a variety of humanoids. It appears as tiny glassy reflective patches on the skin. It is painful and has little treatment available other than pain medication. Spells can numb the pain and slow the spread, however they tend to prolong the time until the shimerscale leaves the body. While not contagious, most peoples shun those with the condition as it is hard to deal with emotionally both for those infected and those watching. Often a local church, witch or other being will take over care of those that they can. Salt mud baths seem to one of the only treatments that reliably dulls the pain and doesn't prolong the disease. The only issue being that other conditions can occur with constant mud submersion and the pain reduction only lasts a few minutes to half an hour out of the mud bath. Many patients take these baths daily and leave the mud on while in linen underclothes, leading the the derogatory term of "mud caked" or just "caked".
Transmission & Vectors
The scariest thing about the condition is that none know how it spreads. The Reflection Beast is commonly ascribed as the source, though it is mythical and not generally considered in serious circles. Various medics and clerics have treated patients for years and there seems no likelihood of contagion during the time in which symptoms are present. Others have suggested that it be a creation of the dragons, psychic cults or tentacle horrors.
Generally the infected will have the scales propagate over the whole body for a period of three years, during which they endure pain that prevents them from doing all but the basic of tasks. After this it disappears within a month and has never been known to return to the same person twice.