What is Game of Thrones' greyscale?

[TMC]

http://www.vanityfair.com/hollywood/...-are-stone-men

The fictional disease has been the focus of recent episodes. As Joanna Robinson notes, "viewers don't need to know every specific of the book version of this disease—there are fatal and non-fatal kinds, it can go slowly or quickly—but they do need to understand the stigma associated with it. The closest real-world equivalent is leprosy, and the environment of fear and ignorance around how greyscale works is why poor Shireen is treated like an outcast by her own mother."