This sickness devastated the Kethran Empire for decades, leaching the life and color from its victims and leaving them with telltale grey eyes and hair—if they survived the initial infection. Most survived only a few months after being infected, with one notable exception being Casia Greythorne, the Fade-Marked main protagonist of the Shadows and Crowns series.
Spoilers from The Song of the Marked and A Twist of the Blade:
The sickness was punishment from Malaphar, who inflicted a curse after one of the rulers of the Kethran Empire refused to hold up his end of a bargain.
Casia survived because her soul was marked by the Sun Goddess, Solatis, and she carries a deep reserve of that goddess’s multi-faceted magic within her.