The Electric Kingdom by David Arnold

• After swarms of deadly flu flies and an unknown virus kill off most of the population, small groups of survivors try carry on in isolation without technology and dwindling resources. 
• In New England, 18-year-old Nico and her dog are sent off on an 8 day mission by her dying father to find a "geological anomaly" that may or may not exist.
• Nearby, 12-year-old Kit and his adoptive siblings are heading out on a journey of their own to find an elusive haven for survivors.
• As their paths converge, they all have a strong sense of deja-vu -- that they have met before and that everything has happened before, perhaps many times.

Grades 7-12

I love a good post-apocalyptic story, and this one is indeed pretty good.  The Electric Kingdom is a little bit of a puzzle-box of a book; it takes some time to make sense, but it's worth the effort.  Beyond being just a straightforward survival story, it delves into science-fiction and/or fantasy territory, which makes things a whole lot more interesting.