this real-world article on parasitic flies, zombified bees, and colony collapse disorder.
What if most zombies aren't created by necromancers or other black magicians? What if they're a natural phenomenon? Throughout history, primitive and uneducated people have always blamed "black magic" or "demons" for things as diverse as epilepsy and alcoholism.
So imagine their reaction when a friend or relative starts stumbling about, having apparently lost all mind and free will, and fighting violently against those who would care for him/her. And what if a scratch or bite from the "possessed" soul could cause the same reaction in others? And finally, when the body seems to have been "put down," imagine what would happen if its head detached and a writhing morass of equally-infectious larvae poured forth.
Addendum: see fleshed-out (pun intended) description on the Besitary page.