Rewind — Marilyn Kaye

No one can identify the cause of the mysterious disease. There's evidence that the bacteria infected human genes as much as a million years ago. Back in the prehistoric age. Back when dinosaurs roamed the land. Back when cavepeople communicated in grunts and gestures. And the bacteria has been dormant -- until now.
Amy's refined genes make her immune to this terrible plague. But when someone close to her shows symptoms of the disease, Amy will do anything to help find a cure. The only way: traveling back to the time when it all began . . .