Another "horrible-alternate-world-scenario" suggestion from Julie the teen librarian, How I Live Now isn't a high tech horror story like Feed. It's scarier because it's far more realistic. The 15-year-old protagonist, a New Yorker with n eating disorder and a wicked stepmother finds herself vacationing with her cousins (one of the kissing variety) in England when war breaks out. Their house gets "sequestered," and children get separated by gender. The rest of the story is of their survival and how they find one another again.
Staying alive is what we did to pass the time. p.155