Spoilers ahead for the most recent episode of Game of Thrones.
Uh-oh — what's that we see on Jorah Mormont's extremities? Could that be ... greyscale? Just when we were worried most for Tyrion Lannister, who could have drowned, it looks as if we should be worried for Ser Jorah instead.
Perhaps we should have seen this coming — after all, the show has been busy foreshadowing that someone would contract this disease, what with Gilly explaining that her two sisters died from it, Stannis explaining that it took "every maester, every healer, every apothecary" in Westeros to save Shireen from it as an infant, and Tyrion himself quipping that prayer was of no use: "You'd have better luck dancing away the plague." So what is greyscale, how does it spread, and what can Jorah do?
There isn't an exact real-world equivalent, although a few come close. George R.R. Martin seems to have based greyscale on various aspects of leprosy, smallpox, calcinosis, and fibrodysplasia ossificans progressiva (FOP), otherwise known as Stone Man Syndrome. People in the world of Game of Thrones seem to think physical contact spreads greyscale, but the disease, which is mostly caught in cold, damp climates, could be airborne, waterborne, carried by parasites, or even, given its supposed origins, something magical.
As legend would have it, Prince Garin, a great warrior, united the Rhoynish people and created the largest army in Essos, a quarter million strong, to resist the Valyrian dragonlords. Garin believed that if they stayed along the river, the water wizards would help them fight the dragon fire — but the dragons were too many, the fire too hot. Watching the slaughter and enslavement of his people, the prince was said to call down a curse on the Volantenes and Valyrian conquerors, where he asked the river to help avenge his people. That night, the Rhoyne flooded out of season, a thick, foul fog fell, and the Valyrians began to die of greyscale. And so, the disease is sometimes referred to as Garin's Curse, and its sufferers — the so-called Stone Men — are exiled to the ruins of Valyria. (In book canon, they live along the Rhoyne, in a region known as the Sorrows.)
Quarantine, however, has not been an effective way to contain it. Greyscale's "terrible, swift" cousin, the "grey plague" or the "grey death," has had at least two known outbreaks in recent history. One in Pentos seemed to have been spread by rats, killing 2,000, according to Illyrio. And another outbreak in Oldtown killed half the city, according to Pycelle. Authorities tried to burn the ships in port and keep a strict quarantine (even killing those who tried to flee) to prevent the spread. The Ironborn also tried quarantine, at least when it came to the treatment of one of Balon Greyjoy's brothers, Harlon, who got to live out the rest of his short life at home, although in isolation. (One theory posits that the Iron Islands were actually originally a colony for those with greyscale.)
Instead of quarantine or exile, the Wildlings took a different approach to dealing with the afflicted: euthanasia. (This may be what happened to Gilly's sisters when Craster dragged them off into the woods on a rope, making sure not to touch them). As one Wildling tells Jon Snow in A Dance With Dragons, "Hemlock is a sure cure, but a pillow or a blade will work as well."
But don't despair, Jorah! As we've seen with the princess Shireen, it is possible to survive this. Stannis didn't go into detail when he discussed his daughter's greyscale treatment, but what might have the maesters, healers, and apothecaries done to help the girl, who is presumably no longer contagious? As we learn from A Song of Ice and Fire, some believe the application of limes (for their citric acid) and mustard poultices (to stimulate nerve endings in the skin), paired with scalding hot baths, especially vinegar baths (for the acetic acid), can halt the spread of the disease. Others believe it necessary to cut off the infected appendages, though that doesn't always seem to work. ("Many a man had sacrificed one arm or foot, only to find the other going grey," Tyrion recalls in ADWD.)
Without treatment, Jorah can expect to have skin that stiffens, calcifies, and cracks — and no longer feels. This is why those who've been exposed to greyscale employ a skin-prick test to see whether their fingers and toes can still feel pain. (Feeling a bit numb? Nails turning black? Time to make your last will and testament!) If it's allowed to continue, the disease will spread across the skin and eventually penetrate under the skin and affect muscles, bones, and internal organs, causing them to shut down. "Death, but slow ... A year. Two years. Five. Some stone men live for ten," a newly infected man considers in A Dance With Dragons. During that time, the afflicted become "feeble, clumsy, lumbering, witless," and eventually, insane.
Now inoculated, Shireen is presumed to be immune from a future outbreak — but there is no other vaccine available in Westeros or Essos. A widespread greyscale contagion could wipe out a population, as Martin once reminded us: "They didn't really know the germ theory of disease ... Disease killed more people than the battles did." Jorah, perhaps you should rethink that trip to Meereen?