The setting is modern America and the poor victim is a 17 year old girl. Apart from the fact she has something that is going to kill her she's healthy and lives a healthy lifestyle. She's fit, eats right, and doesn't smoke, drink or do drugs. She's white and middle-class, perhaps upper-middle class.
What I need is a way to kill her. Slowly but not too slow, and definitely not too fast. Basically I need a disease or illness that basically attacks her in secret and by the time she and doctors realise there's something seriously wrong there's nothing really they can do - unless a miracle occurs she is going to die. They might be able to extend her life a little, but it would be painful and not worth it. Apart from feeling lethargic, in pain or whatever (I am flexible with what symptoms she expresses) she still needs to be able to focus and move about for at least a couple of weeks, before going downhill rapidly and finally passing.
Is there anything that fits the bill? As I mentioned I am flexible with things so any slightly outside suggestions are welcome as well. :)