Beyond the Wall by Madylin Gillett (M_M Gillett)
World war 3 went from a convectional war to a nuclear one. However it was soon ended but the uprising of a lethal and airborne virus that killed over 80% of earths population. Jeffery and his sister live in one of the last untouched societies on Earth, and until now Jeffery never knew about the monsters that were beyond the wall. When the monsters make it into the city, Jeffery and Jessica are forced to find a new safe haven.
