Doctor Blake pulled on her rubber gloves and leaned over the cadaver on the table, which half an hour earlier had tried to kill us. "Strictly speaking," she said, probing the body's skull with a scalpel, "the true Haitian zombie is a victim of the so-called 'Zombie Cucumber'. It's a powerful drug that puts the subject in a cataleptic state. The subject is presumed dead and actually buried alive. The effects of the drug and oxygen deprivation on the brain leaves the subject docile and highly suggestible, and they make lovely lab assistants…"
She paused, as she noticed that we were staring at her.
"…hee! Or so I've heard!"
A zombie is a human or animal corpse reanimated through magic (especially necromancy), super-science, or other means. In many stories and settings, zombies will attack people on sight. In some cases, their bite will infect humans, causing them to become zombies after death as well.
There are three generally recognized categories of zombies in regards to film criticism, to which we add a fourth for RPG purposes.
- Romero Zombie - The slow shuffling brain-eating monsters popularized by Night of the Living Dead and it's imitators. Definitely undead.
- Rage Zombie - The frenzied blood-dripping manic zombie popularized by 28 Days Later and other recent Zombie films. Possibly not undead (certainly not in the case of 28 Days given their tendency to starve to death).
- Voodoo Zombie - Servants of a Voodoo Bokor and not necessarily undead - in some interpretations the bokor has simply stolen the zombie's soul whilst its body remains alive (after a fashion).
- Fantasy Zombie - mindless, rotting, low-rent undead animated by evil magic.
These zombies vary from one another on whether or not they are undead, what causes them, whether they are infectious or not, how smart they are and a variety of other issues. Not least how easy they are to kill - wrong genre savvy PCs may find it deeply embarrassing when headshots (a la Romero) fail to make much impression on a fantasy style zombie.
Revenants also hover around the fringes of zombiedom - although they tend to be more motivated (free willed would be pushing it), and by things other than food (usually revenge).
In addition, a few other subcategories of Zombie may be relevant to gaming.
- Zombie Legion - An army of non-infectionious zombies and Skeletons created by Necromancy.
- Zombie Pirate - Taking the Zombie Legion one step further, sometimes a Ghost Ship will have an undead crew.
- Ninja Pirate Zombie Robot - A trope that happens when writers take things overboard and combine multiple sources or concepts into one bizarrely layered amalgam.
- Our Zombies Are Different - A trope about Zombie categories.
- Zombie Heads - because dismembering a zombie may not be enough.
Just in case necromancy, a curse, or "some sort of virus" aren't scientific enough for you, here's a few possible causes of zombie-ism. Any of these could change via mutation, genetic engineering, or weaponization (for bioterrorism), and result in a zombie outbreak: 
- Creutzfeldt-Jakob disease - the human version of Mad Cow Disease
- Datura Stramonium - the alkaloid neurotoxin used to turn people into voodoo zombies
- Neurogenesis - a process used to repair brain injuries and revive coma patients
- Nanobots - specifically nano-cyborg zombies
- Toxoplasmosa Gondii - a brain parasite that actually makes rats try to get eaten by cats - see Mind Control by Parasites
Potentially a cyborg with extensive wetware installations could remain functional to some degree after the biological bits were dead, assuming some kind of AI direction. For example, where the deceased has had full body, mechanically assisted skeletal reinforcement the tools are already in place for whatever manages the power assist on their skeleton to keep things going without them. Alternatively, a badly corrupted implanted AI assistant could turn otherwise normal people into some kind of rage zombie.
If you are operating or designing an RPG system, there should probably be mechanics for the creation of all zombies (and other undead).
Presumably, we're looking at three basic methods:
- Use the soul of the body's original occupant to control it (probably the "blackest" technique and should require dominating the soul in question somehow): this should give you access to the skills of the deceased to some degree, but may create an unreliable servitor. Probably needs a fairly fresh corpse as well. Very pulp-fantasy (e.g. Clark Ashton-Smith). Prior consent may make a huge difference1.
- Use a third party spirit: depending on the spirit, may yield a varying level of skill, aggression and competence. Has the advantage that you can do this with any corpse you find lying about. Ideal for the mindlessly violent RPG zombie.
- Build a spirit: this may, more or less, be considered a "meat golem" with reliability dependant on the creator's competence. This may not count as "black magic" and is mostly likely to yield a relatively user-safe zombie. Good for "zombie labourer" magitek settings.
What happens to zombies after (say) 28 days/weeks/months should be an important factor - do they rot like a normal corpse and attract scavengers or does that evil magic somehow preserve them? If they rot, do they simply fall apart or do they transform into skeletons? How about environmental damage? Do the "living" varieties age? Starve? Succumb to prolonged damage from fatigue and accumulated injuries?
- News: Mathematical Model for Surviving a Zombie Attack
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- News: Zombie Cockroaches Revived By Brain Shot
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- News: Latin America and the Zombie Factor
Game and Story Use
- Adventure Seed: Texas Red Zombie Chainsaw Massacre
- Zombies make great inhuman menaces - they can't be reasoned or bargained with, and you can only either fight or flee them. The fact that they used to be human (or animal) makes them even more unnatural, and they are usually located within Uncanny Valley.
- Individual zombies are rarely a problem for experienced adventurers, so be sure to use lots of them.
- Here is an example of a scientific writeup of a zombie virus.
- Someone with no particular use for a piece of land might infect it with zombies (probably the one of the contagious kinds) as an area denial weapon (think Ravenholm from Half Life: Episode 1). Actually, even the non-contagious but mindlessly aggressive type might also work. But like all biological weapons (or similar things) it's something of a sword with no handle.