The Shopkeeper isn't feeling well and I can't visit him... he needs medicine apparently... how do I do that?
Go home and do some studying. You'll find a recipe for a serum that cures the disease that his symptoms match.
You'll need to create something as part of the recipe, but you won't flat out get the recipe, just a vague description. You can experiment, or you can talk to the Lizard Priestess who knows how to make the ingredient you need.