Adapt; [v.], to adjust oneself to different conditions, environments, etc.
He doesn't notice it at first, mostly because every day starts off as a constant reminder. There's nothing much to distract him, other than the sounds of his own feet as he moves to where the soldiers order him, and Tommy's nervous chattering as they sit through endless inspections by triage nurses. By the time they settle down in their first quarantine zone, Joel isn't sure how much more he can take. The ache in his chest is like a shrapnel wound, tearing and tearing and leaving nothing behind but a bloody, shredded hole.
He finds Tess bleeding on the pavement, hidden in an alcove on the outside, he realizes he feels it less. This time, it comes as a strange sort of nostalgia, a burning in his lungs that feels like nothing more than a memory. Joel tries to help her up, despite her swears and protests and the bruises she gives him, and while he helps her clean and wrap her wounds, he knows she will stay with him. He notices, then, that the longer Tess is with him, the less he feels the burning in his lungs or those gashes inside himself that feel like broken glass.
He wakes up in the middle of the night, drenched in sweat. The moon is high in the sky, and the girl beside him is curled in a ball under her plaid, flannel jacket. Ellie has been with him for two and a half years, following on his trek across the continent; surviving. Around them, there is eerie quiet, nothing but the sounds of the gentle breeze and their own breathing. Joel looks around, and that's when he knows. He has not felt the pain in months. He doesn't remember the way the assault rifle sounded, or the way he spun and tumbled down the hill, or the fear that flooded through him when that flashlight shone in his eyes. He can't even remember her face.