"I've been sleeping a lot," Stiles says. "I'm not eating. I don't have any energy. My mind isn't working as fast as it used to. I feel run down. Breathing hurts, sometimes. My hair's started to fall out. My muscles spasm. And before you ask: yes, they've run tests. I went to the hospital over in the Valley and they checked everything -- ruled out everything. The most they can come up with is Chronic Fatigue Syndrome but I don't fit all of those symptoms and --."
He stops there, looks down, away from Peter's eyes. "What?" Peter asks, gently.
It takes a moment for Stiles to steel himself, to meet Peter's gaze, to say, "I know what it is. Why I'm sick, I mean."
"Please, Stiles," Peter says. "Tell me."
When the nogitsune and Stiles split, their bodies got mixed up and torn apart, leaving Stiles to inhabit a completely magical construct. He needs an anchor to the physical world in order to survive and when he can't wait any longer, he goes to Peter Hale.
- Part 1 of Everything