I think this would have been a good horror story on its own, but it is the way of genre to externalise metaphors as monsters, and so the lonely forests at the border with Canada are infected with creatures somewhat like the Alien, which lay their eggs inside the human body, inside the uterus. These parasites have a psychic defense system which induces religious mania in the community, making them objects of worship (and there is a second major plot coming in at a tangent to this one, but no need to explain it).
It would make a good Hollywood film. There are three strong male characters: a black doctor educated at the Sorbonne, a Pinkerton detective who is a devout Catholic, and a Montana farm boy, who work together to fight evil. They are the 'fittest' if you wanted to classify people in that way, but each for a different reason overlooked by the eugenicists. The Klux Klan are active in the town, and like the monsters, they clothe themselves like ghosts.
The female characters are objects rather than subjects, unfortunately, tending to passive obstetric disaster, with the exception of one outrageous villainess, who I thought was jolly good fun. It's well written, and the plot works sensibly enough. It's more yucky than frightening. I wasn't scared at all. I listened to it on audio, and the reader did a good job, injecting a nice juicy irony into the delivery, and at times it was almost a black comedy if that seems possible.
Why all these different horrors come together in one place isn't all that plausible, but never mind. Probably I would have been happy with a more realistic historical horror, about eugenics, but in so far as horror stories need to make monsters out of abstractions, this does a good job of it.