Okay, in this one nanobots are used to make the zombies, but no one knows it.
At first.
Starts with a group of people in a fenced in farm area, old Marine sniping off the top of a grain silo.
Nanozombies start evolve, use spears and tactics.
Something like: The wild, out of the wild or something along that line.
4 books in all.
Thanks!