From the introduction: One of the most bizarre symbiotic relationships ever witnessed among the undead has been the Infested, these are zombies that are carriers of not only the zombie virus, but of other, smaller creatures that take residence inside their bodily cavities. The size and threat these zombies pose varies tremendously by the zombie’s size and how many creatures that live inside it. It is assumed that these zombies were created when a body being feasted upon reanimated with the vermin still inside it. Instead of fleeing, they take advantage of the situation by swarming out and attacking the same target the zombie attacks, and collectively feast upon the remains. The swarms of rats and flies that live inside them can be seen writhing just beneath the surface of the skin.