I promise to give you comment back then, Kyucchin .. So, here i am.
I have told you that there is some grammatical errors. So, in this post i will not mention it as a commentary point.
Let's see ....
First,
Kyuubi no Shinigami wrote...
Another loud cry,
like the growls of an underworld animal
Your analogy here is kind of absurd if you ask me and somehow i find it exaggerating.
There are many sentences that i find exaggerating (i will just mention one for example) but i can safely assume that you try to build intense atmosphere here ?
Second,
Originality .. not want to sounds like an ass. But Zombies story is overused lately. With prologue like that i cannot help to think that this is a fanfic for L4D lol.
What i said above is not the cons of your story. There are something that you need to know and find the solution to fix it.
As per saying, you can erase 'Originality' by adding something new (no, not by adding some F! member in your story) and come by your own idea.
Your story is most likely using the same concept as Left4Dead.
Aside from my ranting, your story is not that complex nor ' heavy' .. So, people in majority can read and enjoy it easily.
If i could suggest something ..
.> Add more funny/hilarious events.
Make readers laugh, scared and excited at the same time ... sounds like a good idea.
.> Your story's (maybe) strongest point will be characters.
So , put your focus into character personality and its development while not ditching the flow of your story.