There are more and more immunocompromised children every year, this is because childhood cancer is no longer always terminal.
She has probably not caught anything because her classmates are vaccinated, as are most people she encounters.
By your own admission the one time she was in a the presence of a childhood illness she caught it. Maybe she has been very lucky and not encountered any of the others.
But here's the kicker, no matter how strong she is, how effective her immune system is if she does catch one then she is on her own.
She might as well be in the 1950s, measles has made a comeback and could kill her, or just leave her deaf.
Rubella might not have much affect but she could infect a pregnant woman.
Will you be laughing then?
BTW I hope she remains lucky, I really do.
I've never needed a seat belt, I still wear one.