Katie nods. She's seen that a lot in the hospital, how much harder every subsequent illness can be on a parent. Even the ones who are chronically ill, their parents never get used to it. They learn how to handle it, but the worrying never eases. "I'm a pediatrician," she says. "I work with kids who are seriously ill all the time. And still every time Jamie has so much as a cold, I'm terrified. I don't know what I'd do if he'd ever had something like diphtheria."

She smiles, watching the pair of them at play. "She seems healthy now. Strong."