Andrew Fukuda 
The Trap [EPUB ebook] 

After barely escaping the Mission alive, Gene and Sissy face an impossible task: staying alive long enough to stop an entire world bent on their destruction.
Bound on a train heading into the unknown with the surviving Mission girls, Gene, Sissy, David, and Epap must stick together and use everything they have to protect each other and their only hope: the cure that will turn the blood-thirsty creatures around them into humans again. Now that they know how to reverse the virus, Gene and Sissy have one final chance to save those they love and create a better life for themselves.
But as they struggle to get there, Gene»s mission sets him on a crash course with Ashley June, his first love . . . and his deadliest enemy.