Sickness/Disease for a 14/15 year-old girl?

I'm composição literária a story where the girl, who is in high school, has just been diagnosed with some sort of sickness or disease. I'm not completely sure of the plot yet, but the story will have her dealing with telling her friends about the diagnosis. I'd like to have her die quietly at the end, preferably while she's asleep undergoing some sort of operation/treatment. I've been looking around, but I haven't been able to find much...

If possible, I'd like the disease/condition to:
-Let her live awhile, 6 months to a year?
-Have some sort of treatment/surgery that she needs to undergo
-Have a partial chance of surviving the operation/treatment

Like I said up there, I'm still working on the plot, but I'll need some sort of sickness to research and work from in order to really get it down.
Thank you! :)