Survivors

The curse is easy to remove in daylight, but it's trickier at night or somewhere with no sunlight reaching it. Fortunately, even magical sunlight counts for weakening the curse, so simply taking the girls outside should do the trick, or to a magical greenhouse lit by an artificial sun.

The elemental parasite is water and fire, mixed together. It's growing by draining the elemental water and fire from Willow, though fortunately the progression is usually quite slow. The progression on the parasite is 1d6 days per stage, though Willow rolled a one to begin with so without some magical disease-curing effect she's due to start spreading the parasite tomorrow.
The first being sneezing, which can transmit the parasitic elemental to new hosts.
Stage 2, inside of the mouth and bodily fluids turn glowing neon blue
Stage 3, skin starts turning into stoney patches, small tendrils start growing
Stage 4, teeth fall out as burning fangs grow in, face and jaw rearranging to accommodate. Body mostly turned to stone armour to contain the element.