I like Zmerr's idea how nature kind of collapses on itself. In Undead Nightmare (I may reference this alot in the future), animals get the zombie disease as well and the birds turn into bats. We could have that happen, where the "zombie" disease affects every form of life (excluding plants and what not), therefor nature kind of collapses on itself, as we cannot eat the animals and the plants would be rotten. That, and we can have the "zombie" animals destroy alot of the plants and crops as well.