the enemy is us

Zombies are definitely a catch all creature.  There is really no way to make them sexy or safe (I have seen them funny, but that comedy also involves some fear and disembowelment, so still--not safe), so they can stand for whatever fear we might have.  They are the other--standing in for racism, terrorism, consumerism, fear of disease ... the list does go on.  Most importantly thought, zombies are about us.  

There are a few things that I think are imperative to stories involving zombies:

1)  The survivors, the humans, are trapped in some way and forced to band together--whether they like it or not.  Being trapped isn't limited to being stuck in a single location like a farmhouse or a mall, it might also being trapped outside where the danger is omnipresent.  The key here is the being forced to rely on others, people who may be familiar or who may be strangers, to survive.  This is where we get to the juice in most zombie pictures because this is where we see both human frailties and nobility butt up against each other--pettiness becomes obvious, small details tend to carry greater resonance, and each moment could be the last one.  We see humanity in a microcosm and we see how humanity can fail and succeed depending on what elements of society we hold onto and what elements we abandon.

2)  Zombies are not only a fear of things that are different from us, but also of us.  Zombies are us.  They are family, friends and neighbors.  There is the constant threat that insufficient diligence or a momentary lack of focus can cause you to become one of them.  With other creatures such as vampires and werewolves, there is a continued consciousness:  you can still retain parts of you, even if part of you is now also a monster.  Vampires have an insatiable hunger, but they still retain personality.  Werewolves change into a beast, but at some point they still change back.  There is a duality in these creatures.  With zombies, it's an empty afterlife:  you are condemned to wander about without consciousness or a soul and eating other people without the promise of life after death, or even a peaceful nothingness after death because you are aware of what your body will be doing after you lose control of it.  In addition of the fear of what happens to you, it is also the continued pain of the multiple deaths of the people that you know.  You are now faced with their death, their undeath where they are attacking you, and the prospect of killing the body of someone you may have once cared about.

3)  There is the threat of plague.  Zombie origins (since Romero reinvented the creature) are deliberately vague.  No one is ever certain why the undead are suddenly everywhere, shambling or in some cases running after you.  So there is the threat of this disease of zombie-ism spreading unchecked without a cure.  This element made it easy for people to consider the "28 Days Later ..." and "28 Weeks Later ..." movies as zombie movies, even though these movies are straight up plague/ disease movies.  The metaphor still hangs comfortably around the zombie's shoulders--it is something that you can catch through a bite, through blood, through saliva.  That the zombie is decaying is also a signifier of disease and plague, the body is falling apart, and as I stated in my second point, it's a body that might be recognizable.

4) Finally, there is the fear of the other.  In "Night of the Living Dead" it starts with a few zombies around.  It seems like the threat is something that can be dealt with, it's small and manageable, but then quickly the threat multiplies.  They may be slow moving and without reasoning capabilities, but they are a legion.  They can't be easily killed because the bodies are already dead, and the numbers are suffocating.  The other--be it a person of another race, religion, or government--comes into the community.  One person is not a threat, if the community doesn't like what the other is presenting, the other can be dealt with, but as more arrive, suddenly the other has the majority and you are the one who is threatened.

So yeah, zombies are a catch all for what frightens us.  It's about situations getting quickly and completely out of our control.  It is our culture mirrored in a grotesque way and it is the worst of us magnified--on both the sides of the undead and the survivors.  Notice in any zombie movie, the remaining humans are always undone by themselves--greed, pettiness, carelessness, despair.  Zombies are often just the surrounding context for these human dramas to play out.