Attempting to put it all together...
Fading (Su): Supernatural disease—bite, Fortitude DC 14, incubation period 1 minute; damage 1d6 Str. The save DC is Charisma-based.
Unlike normal diseases, fading continues until the victim reaches Strength 0 (and dies) or is cured as described below.
Fading is a powerful curse, not a natural disease. A character attempting to cast any conjuration (healing) spell on a creature afflicted with fading must succeed on a DC 20 caster level check, or the spell has no effect on the afflicted character.
To eliminate fading, the curse must first be broken with break enchantment or remove curse (requiring a DC 20 caster level check for either spell), after which a caster level check is no longer necessary to cast healing spells on the victim, and the fading can be magically cured as any normal disease. A dimensional anchor, dimensional lock, or similar effect stalls the progression of the disease for as long as the spell remains in effect (and the victim remains within the area of effect, in the case of dimensional lock).
As the victim fades from view on the Material Plane, it slowly gains substance on the Plane of Shadow. Once the victim reaches Strength 0 and dies, it has fully materialised on the Plane of Shadow, where the shadow wolf can eat its meal in peace.