The Phoenix Descent — Chuck Grossart

The year is 2025. Astronaut Caitlyn “Sif” Wagner and her team emerge from stasis to discover that their Mars mission has gone terribly awry -- the crew has run off course in space and, they suspect, in time as well. Their damaged ship returns to an Earth reduced to overgrown cities and blasted terrain. Yet humans have somehow survived, living in caves, foraging at night, returned to a tribal existence. Sif meets Litsa, the fiercest warrior in her tribe, and learns a horrific truth: the planet is overrun with the Riy, a swarm of spore-releasing revenants intent only on spreading their infection.

But even as Sif and Litsa unite in combat, they soon realize that the battle against the Riy is only one stage on which they must fight the war for humanity's survival.