Do you see the natives? Their lack of colour, how regardless of how long you spend looking upon them, you cannot recall their face once you look away? Even the light is a bit duller, the darkness a bit darker, and the places less traveled are completely devoid of life and colour altogether.

[ This much, he knows firsthand. The rest, though - ]

We are told it's an illness, a plague upon the realm. And we are brought here to cure it.