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Overview
Greyscale makes the surface of the skin harden and calcify, but in the early stages it is still fairly thin and brittle, so the body's movement makes the hardened tissue split into a network of cracks. The pattern made by the hardened and splitting skin is similar to cracks made in thin ice, and in later and more severe stages look somewhat like patterns of scales. In later stages the hardened flesh becomes mottled grey and black as it dies, resulting in the flesh appearing to have the texture of cracked grey stone, or like grey reptile scales - hence the name "grey scale". The discoloration comes in later stages: Shireen was cured in early stages before the disease could spread beyond her left cheek, before it could become discolored.
[4]
Greyscale kills slowly, often taking many years to progress to a terminal state. Long before that, in the mid-to-late stages of the disease, the infection will spread to cover all of the victim's skin from head to toe, leaving them in agony. An infected person can survive like this for years, living in misery as one of the Stone Men. In the very final stages of the disease, (perhaps years after the skin is completely covered with it), it starts attacking the body's internal organs, hardening them as it did the outer flesh. When it spreads to the brain, the deterioration can result in violent insanity. Most people rarely see these very final stages, however: only some of the Stone Men who have been quarantined off in ruins such as Valyria for many years have time to deteriorate to such a state. Otherwise, Stone Men are not thought of as violent but are pitied for their misery - though still shunned for being infectious.
[5][6]
Greyscale is extremely infectious, spread through touch contact with an infected person, or even spread on unsterilized objects that have been touched by the infected. A single touch from an infected person is enough to spread the disease to another person.[7]
Greyscale is relatively uncommon in
Westeros, though it does occur from time to time both in the
Seven Kingdoms and
Beyond the Wall. It is far more common, however, in the
Free Cities and other parts of
Essos. The number of people infected by greyscale is large enough that colonies are often established in old ruins such as at Valyria where Stone Men are dumped off, to keep them quarantined from the rest of civilization.