What terminal illness is this?

I am currently working on writing a book, and the main character is going to be diagnosed with and eventually p away from a terminal illness. The only problem is I don't know which one. The way the plot is progressing, the illness isn't going to change her physical appearance except weight loss of course. The only real symptoms I have are her being very very weak and being unconscious for long periods of time. Are there any illnesses or cancers that you think would work in this senario?? thanks!!