In my meta setting, dragons cast "magic" (read: nanomachines) through their breath. This magic can be anything from a healing spell to fire or a potent poison.
It just so happened that in the kingdom of [undefined], a plague popped up in the countryside, where a dragon happened to live. The king sent Red-crossed Knight and a gryphon to investigate the case, who quickly found and defeated the dragon.
Though the knight's original plan was to kill the creature where it stood, the gryphon convinced the knight that maybe they should investigate a bit more, because something felt fishy, plus the gryphon wouldn't let anyone be executed without a fair trial.
Of course, since I'm making the story, I already know that the culprit is a necromancer who is poisoning wells with gamer girl bathwater, but that still wouldn't necessarily save the dragon's life. You see, dragon blood is able to cure any disease, but the dragon has to be dead for it to work for some reason.
Otherwise they could just
ask force the dragon to donate his blood and considering they already beat him up, and because he's more of a scaredy-cat, he would still cooperate. Besides, killing him wouldn't get you that much extra blood, but it would certainly invoke the wrath of a mama dragon and The Manager himself, from which you can't come out with a net gain, in fact, you would be lucky to get out of such ordeal with a bone left unbroken.
So, the whole point is that they would have to kill the dragon and use his blood to cure everyone if they can't find the source of the plague.
But why would the dragon have to be dead for it to work properly?
Magic here simply means nanomachines doing stuff. The magic inside the dragon's blood is just an immune system that has been on the field for so long that it has seen and can deal with just about anything.