George RR Martin’s Greyscale certainly echoes the woos of leprosy that ruled in the Middle Ages with its skin lesions that often provoked feelings of terror and repulsion, forcing those affected to live like social outcasts until they died.
In addition to leprosy, fibrodysplasia ossificans progressive is another horrific disease similar to Greyscale insofar as it turns the damaged soft tissue in the body into bone, eventually “freezing” the body and turning it into “stone”. Fibrodysplasia ossificans progressive is extremely rare but nonetheless resembles Martin’s Greyscale rather vividly.