By the end of book one it was revealed that Dr. Kate Lovato's bio weapon, which she designed to destroy the zombie hordes, was quite effective, but also not effective enough. Doing the math we know that 80% of the human race had turned into flesh craving zombies before her weapon was deployed, and that her weapon killed billions of them, but it also left behind a resistant strain of Variants. The real question is, how many Variants are there and what are they capable of? Do they number in the thousands or the millions? Can this new threat of intelligent, agile Variants be contained or is it actually a bigger problem than the billions of mindless zombies that were wiped out?
Extinction Edge - the real threat emerges in book 2