well ... she's an orphan whose relatives are unknown, her previous master used acid to burn her, and enjoyed her creams, she was barely fed anything at all, and she slept on the cold stone floor, and majority of the time she remained naked for her "master's pleasure" and once the master "supposedly" died, (he had a lot of enemies so vendetta is possible) the merchant took Sylvie from him and treated her as another product for selling.
based on the prologue the merchant got really sick, so he dropped on the road, if it wasn't for Gris "the game's canon name for the town's doctor" the merchant would have died, and he was in debt for the doctor, and while he was healing, god knows what happened with Sylvie..
so you are telling me that .. an abused, unfed, cold, weak, PTSD, traumatized, immune deficient, girl who found some rags and DIY something from them to cover herself, without ever having any healthcare at all this girl is not supposed to get sick, as her body goes under radical biological change from being abused, into being tutored what "feeling" is like? as in "literally what FEELING anything" is like?
also throughout the entire game she only gets fever ONCE, and after that, she's fine forever.
by the way, according to the canon lore of the game, Sylvie will never age, never get married, never have offspring, and all these fan fiction arts, are non canon. YES Ray-K once thought about making an adult version of Sylvie, but that concept art went on to create his latest creation from it "courtship dream"
Daily life with a slave: Teaching Feeling, is an endless game, pretty much.
and it's the EASIEST ON THE EYES game, from the entire collection of "Freaking Charming" collection.