I go for the bubble. Of course, her extraordinary biodata could explain it. Or simply the pasage of time. Perhaps the resistant gene was bred out of successive generations. The genetic remnants of Earth bereft of a gene or two.
Immortality is too good for that girl.
On another level, some reviewers have commented that this book is about as scientifically accurate as Barney the Dinosaur. Well, I've got to tell you. To me a star is a star. A planet is a planet. If a wormhole pops up then I can go with it.