Nico is immortal and somebody at camp, not knowing this, poisons him, but he never realizes he is poisoned because he’s immortal and it doesn’t affect him at all. It’s like it never even happened, as he can’t be harmed by poison due to his immortality. But then Zephir, a Vampire, needs to feed off Nico, not knowing he’s been poisoned, and they get poisoned from the traces of the substance still in his blood. So now, Zephir is the one with their life on the line as the poison flows through their veins, and Nico has no idea what it is or how to find out before it’s too late.