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The story was about a young girl who ends up alone after some major disaster - a disease I believe. She thinks she is alone but eventualy discovers that there are others like her, other Alphas. She meets a young boy about her age who also believed he is alone, so they team up and travel across America to a location where they believe others like them are headed. Alphas are the next stage of evolution - smarter, stronger, healthier. I have searched the web but can't find this book. Any help would be appreciated.