Rocket could launch without zombies and then one day just patch them in and watch what happens. Imagine one day you're scavenging in Stary and come out of the market. The only threat known to you at the moment is other players. You see movement down the street and decide to check it out. As you quietly move towards the suspected player, a zombie comes around the corner instead. It immediately becomes aware of it's potential meal of brains and flesh, and howls at you as it chases you down. As you are running away, the howl of the first zombie alerts a wandering horde of zombies in a nearby forest, and Stary becomes completely overrun blocking your escape.
