Greyscale is a typically nonfatal disease in the Song of Ice and Fire series. It is first introduced in Stannis Baratheon's daughter Shireen. When it infects children, greyscale generally leaves children malformed and disabled but alive. However, in A Dance with Dragons, it is revealed to be generally fatal to adults. The disease is contracted by touch and slowly turns the flesh (small patches in children and the entire body in adults) of the victim to stone. It is said that the disease also drives its adult victims insane.
In A Dance With Dragons, greyscale is encountered by Tyrion Lannister as he sails through The Sorrows, a stretch of river. The boat the Shy Maid is attacked by several men afflicted with greyscale as they sail through the Sorrows.