This is probably not the answer you're looking for, but I am also stuck the same way as you. I just haven't explored "the woods" as much, I suspect there might be the cure there for the parasite
I read on another forum page (I have a reading comprehension disability so take this with a grain of salt) that it is indeed in the mansion, but i can't explore it because I only have one key, and the other key that you get which takes you to the second floor is accessible from said West key, so I'm just lost