Castle could only breathe again when he secured Martha and Alexis in the safe house. They begged him to leave the virus-laden New York City, but he could not leave his heart. She was still there; he knew it like he knew the ink in his veins. If she was not, he wished for her sake she were dead.
When Beckett killed Ryan, or the creature that used to be one of her own, she shot him in the face. No remorse struck her eyes, but the unbidden grief threatened her lungs with shrapnel. She survived on a bare matchstick of relief that burns whenever she turns a body to find that it was not Castle.
When they found the other, there was no circling. They yielded at once, liquid fire collapsing their bones no other virus was capable of, ravenous in a way that even the undead could not know.