Activity

I need help finding some sort of a dangerous disease or sickness for one of my book ideas. I’m trying to find something that is not easy to tell the symptoms of and is possibly fatal and somewhat untreatable or something like that. In my story, the girl gets this disease or sickness and she dies before the doctors can fix it or do anything about it and no one can really tell that’s she’s sick and she doesn’t even think she’s that sick. Any ideas for what sickness she could have?