Take My Breath Away

Jimin had struggled to breathe his entire life, but he never imagined that his least favourite person in the world, Jeon Jungkook, would be the one to properly take his breath away.

or

Jimin has cystic fibrosis, an incurable genetic disease that affects his lungs, and nobody knows. That is until Jeon Jungkook, the bane of his existence, finds out. Jungkook has a secret of his own and they bond over their shared misfortune. As the two broken boys fall in love they heal each other in ways they would never have imagined, but when Jimin's health starts to deteriorate they realise their time together might not be as long as they thought.