I'm here to tell you the truth. Zombies are already here. The only reason you haven't seen a hoard stumbling across your front lawn, is that zombie parasites have yet to sufficiently adapt to human anatomy in a way that allows infection of our species.
Or have they?
In my latest book, Matriculated Death, I explore a fictional way for zombie parasites to cycle between humans and insects, turning both species into mindless tools of destruction. Here's a sampling of the zombie parasite life cycles already happening:
1) The nematomorp worm. Grows inside crickets and spiders. Reaches a certain stage, and commands the host to drown itself. Which the host does, because it's been zombified by the parasite.
2) Zombiefied snails. They beg birds to eat them. Also, very weird eye stalks.
3) Zombie cat owners! Yes, I'm offering you a possible example of zombified humans. Read the article about Czech scientist Jaroslav Flegr, who is convinced that a parasite from his cat's feces is driven him to self-destruct.
Don't fear the zombies that could come. Fear the zombies that are here.