The Turned

So several people have mentioned turning into a zmobie only by infection or Zombie bite…but that’s not the standing zombie lore as I understand it to be. From the latest zombie movies and shows everybody is already infected by some virus that incubates in the living but reanimates the dead. In Unturned I always saw infection is a realistic way of dying in a suvival situation but not necessarily related to the cause of the zombie outbreak.

Is it explained somewhere in Unturned how the Zombie outbreak occured? Is it something all humanity has like The Walking Dead or Z Nation or is it something else?