According to Kirkman, zombies were never a thing in Rick's world. The infection may have created what we know as zombies, but the characters on the show had never heard of such a thing.
During an appearance on Conan, Kirkman said that he wanted to separate the world of The Walking Dead from all other types of zombie fiction. Basically, in this reality, zombies were never even an idea that someone had.
If you lived in The Walking Dead world, things like Night Of The Living Dead, or Nazi Zombies, never would have existed.
