A few years back, the Delers released a virus into the air that made 'zombie' like creatrues. It is a parisite that clings to pretty much any living thing (Including plants) and turns it into a scary beast like creature. In most animals, they will grow a claw on one of their limbs that releases spores into their next victim on impact. That's how it spreads.
The zombies can only be killed with a special bullet that has a toxin in it that is extreemly toxic to the blood. ( A special snake venom mixture, and the recipe is classified ) The bullet injects it into the veins of the zombie and hardens the blood.