The Passage by Justin Cronin
The U.S. government once again try to create super-soldiers. This time with the help from the virus of a bat found in the Bolivian jungle. It does not go well and the vampire-like beings get free and spread like wildfire and kill everything. The only one still alive and human after the experiment is Amy, a six year old girl. A century later the humans live in walled of compounds with the light shining all night, trying to survive when they get a new visitor from the outside world. A fifteen year old girl with the name Amy.