That's interesting actually. I never heard them mention anything about a disease. Have you been reading spoilers and your mind is filling in blanks? All I got from it was that Brainiac has done something to alter Chloe and as far as Clark's concerned that can't be a good thing, hence his need to try and fix it. I don't think Chloe knows anything about it killing her, if she did I think she'd be more eager to fix it. No matter how special it makes her feel I don't think she'd want to die for it