Selenium's explanation was too summarized. It wasn't just any disease she just happened to have the misfortune to pick up, it was an elf curse from her clan that slowly turns the victim into a tree. Don't remember if it's used on criminals of a certain level, people attempting to leave the clan for good, or elf supremacists who hate half elves. Not sure how they apply the curse but I think the idea was that if victim couldn't contribute to clan as a person properly than would become part of the forest or something. Also, Apple was (as Miracle Max would say,) mostly dead from the arrow to the chest but Selenium just barely saved her by tossing her into Polka's healing spring water. However, while the water saved her life she became comatose.
Due to the two of them helping out Polka village a lot with their herbs and stuff Apple being wounded quelled the racial anger between Polka and the elves a lot, and the Mayor is letting Apple stay in one of his rooms while his wife helps look after her.