What Exactly Is Greyscale On Game Of Thrones?

Photo: Courtesy of HBO.
The Game Of Thrones season 7 premiere, "Dragonstone," left fans with a lot more questions than answers. Why did Daenerys Targaryen (Emilia Clarke) get so little screen time? Is Jon Snow actually the worst? And, seriously, what in the Seven Hells is greyscale, and why is Jorah Mormont (Iain Glen) basically a poorly moving rock now?
The last time we saw Daeny's BFF in season 6 episode "The Door," Jorah had just told his Tagaryen queen about his illness. She took the news as well as a queen who doesn't want her city to die of a highly contagious plague could take it, demanding her longtime friend go find a cure. Since then, Jorah's medical status has been a mystery.
Now, we know the Westeros native is worse for wear in his home country's Citadel, and his arm has essentially turned to stone. To understand what's going on with Jorah on Game Of Thrones, we gathered every single thing you need to know about greyscale, from what the symptoms are to the leading theories on how to cure it.