Slow Burn by Tracie Douglas

Brothers Drew and Tyson Hawkins embark on a search for each other, and find themselves confronting more than just their inner demons.
Kate Reilly is a scientist with a secret. A secret that could either destroy her, or be the key to their future existence.
Individual lives collide, becoming intertwined out of necessity. Friendships are formed and romance blossoms as a group of survivors fight their way to safety.

In a world where the dead aren’t quite dead, and life as it was has ceased to exist, the only options are to survive or to die. Faced with the choice of allowing your fear to be your downfall or your saving grace, all you can do is hold on and keep going before your entire existence becomes nothing but a graveyard of memories.