Aside from briefly mentioning Dr. Herbert West's experiments and their 1940's offshoots, we haven't really covered the various origins of zombies. There are several. Being raised as a mindless slave by a voodoo practitioner is well-documented and even examined in mainstream academic literature.
Fortunately, these zombies tend not to be as hungry or as contagious as some of the other varieties. They can still be dangerous, so it's good to learn more about them and how to identify them.