This is a script idea for a film that I've been toiling with in my mind for a while.
Two friends have known each other since childhood. One has always been a bit weaker and slightly ill with an unknown virus. The other friend makes the promise to one day find a cure to the other's condition and studies to become a doctor.
Twenty years later, the doctor gets close to understanding the nature of the illness and finding a cure. Both are excited at the prospect, but as the cure is applied, the friend seems to gradually get weaker. The doctor scrambles to figure out what went wrong, but each day that passes, the friend seems to slowly lose grasp on the world. Eventually the friend becomes bedbound and the doctor caring daily for the friend.