Go Home and study about the sickness in your notebook. You will learn Spectral Sirup (the cure) and you will see a description for Heat Occisia, an ingredient you have to craft first to make Spectral Sirup.
If you cannot figure out Heat Occisia from the vague description in your notebook you can visit Othra the lizard priestess in the Ruin. Talk to her and you will learn the recipe.
Craft Heat Occisia then craft Spectral Sirup, then visit the shopkeeper and give him the medicine.