In a Song of Ice and Fire, what is the etiology of Greyscale?
The real-world diseases discussed in What's the real world equivalent to greyscale disease? all have very different degrees and mechanisms of contagion, so is there more than one way that one can contract/get greyscale?
The books seem to indicate that the primary vector for transmission is physical contact - and it seems that only a moment's touch is needed. Of course, that idea of sudden touch could just be the perception of the characters themselves - in much the same way that we once thought diseases were carried by "bad airs".
If this kind of contact is needed, then I suppose we are looking at something that is likely blood-borne.
I don't think there is any evidence of an airborne vector in the books. Out of those people who have had contact with sufferers of Greyscale, only those who actually made physical contact (even then, not all of those characters) have been seen to show symptoms afterwards.
Most characters in the book seem to believe that physical contact is what causes you to end up with Greyscale, although this does not seem to be entirely true based on a Dance with Dragons.
Tyrion gets dragged underwater by a Summer Islander afflicted by Greyscale and he seems pretty much fine. While characters suggest he might be infected internally, there is no real sign of it. On the other hand, Jon Connington only drags Tyrion out of the water and starts showing signs of Greyscale in his toes soon after, even though he wore gloves.
To me, that seems like there is at least an additional requirement for contracting Greyscale. Perhaps a certain genetic makeup makes you suspectible, but simply being touched doesn't seem to be (only) reason to get it.
Tyrion believes that Greyscale can be contracted through touch and through breathing in damp and cold areas. It's also said that Garin's Curse (another name for Greyscale as explained by Tyrion) can be contracted through breathing in fog.
"We'd do well not to breathe the fog either," said Haldon. "Garin's Curse is all about us."
The only way not to breathe the fog is not to breathe. "Garin's Curse is only greyscale," said Tyrion. The curse was oft seen in children, especially in damp, cold climes. The afflicted flesh stiffened, calcified, and cracked, though the dwarf had read that greyscale's progress could be stayed by limes, mustard poultices, and scalding-hot baths (the maesters said) or by prayer, sacrifice, and fasting (the septons insisted). Then the disease passed, leaving its young victims disfigured but alive. Maesters and septons alike agreed that children marked by greyscale could never be touched by the rarer mortal form of the affliction, nor by its terrible swift cousin, the grey plague. "Damp is said to be the culprit," he said. "Foul humors in the air. Not curses."
A Dance with Dragons, Tyrion V
The following passage from The World of Ice and Fire seems to imply the damp fog and waters of Chroyane killed all it's inhabitants of greyscale.
At Chroyane, the cage was hung from the walls, so that the prince might witness the enslavement of the women and children whose fathers and brothers had died in his gallant, hopeless war...but the prince, it is said, called down a curse upon the conquerors, entreating Mother Rhoyne to avenge her children. And so, that very night, the Rhoyne flooded out of season and with greater force than was known in living memory. A thick fog full of evil humors fell, and the Valyrian conquerors began to die of greyscale. (There is, at least, this much truth to the tale: in later centuries, Lomas Longstrider wrote of the drowned ruins of Chroyane, its foul fogs and waters, and the fact that wayward travelers infected with greyscale now haunt the ruins—a hazard for those who travel the river beneath the broken span of the Bridge of Dream.)
The World of Ice and Fire, Ancient History: Ten Thousand Ships
