Zombie Bible

Life cycle of a zombie
Zombies are driven by primal instinct to seek out human flesh. This instinct is what creates more zombies. A person who has been bitten by a zombie (hard enough for the zombie's teeth to break their skin) is doomed to become one as well. Nothing can reverse this process.

Once bitten, the person falls ill within a couple hours. They suffer a long and painful sickness characterized by intense fever, chills, stiffness, nausea, vomiting, and delusions. The sickness is 100% fatal and runs its course in 1-2 days. After dying, the person reanimates as a zombie. The time from death to reanimation can be variable (ie we can set it to whatever works best for our story)

Physiology of a zombie
Due to rigor and decay, the typical zombie moves fairly slowly, lurching and stumbling. An exception to this general rule is zombies who have just recently reanimated, who may be fairly quick (albeit still uncoordinated).

A zombie is incapable of feeling pain and will do little to prevent itself from suffering physical damage. A zombie can only be killed by catastrophically damaging the brain (see section below, "killing a zombie.") Of course, sufficiently damaging extremities may incapacitate one to the point that it can no longer move, but it will still be "alive."

Zombies are clinically dead human beings. A person who has been turned into a zombie can never be "cured" or brought back to life.

The typical zombie displays strength roughly commensurate with the strength that it had when it was human. A group of zombies working in tandem can easily break down a door, tip over a car, or rip a person in two.

Zombies hunt primarily by sight and sound. A zombie's eyesight is about as good as a living human being's, and their sense of hearing is a little bit better still.

Zombies do not die of starvation and they do not tire. They are always active.

Behavior and intelligence
Zombies are driven first and foremost by a desire to consume human flesh. A zombie that has caught sight of a human being will not stop pursuing that human until either it or the human is dead -- or until the zombie is attracted to the presence of some other human. For instance, if a zombie has pursed someone into a building, it will wait outside that building forever, until the human comes back outside or it is distracted by something else, (ie the sound of gunfire).

A zombie that is not actively pursuing a human will wander about at random.

Zombies are strongly attracted by sound. There are two kinds of sound which attract zombies:

1) The moans of other zombies. This means that zombies tend to swarm into massive hordes, sometimes numbering well into the thousands. Going back to our example from above, it means that one zombie waiting outside your door can quickly become a dozen, then a hundred, then a thousand.

2) Sounds created by humans. Zombies are instinctively attracted to any sound that is likely to have originated from human activity. This means they're attracted to the noises of running engines, gunfire, power tools, human speech, and so forth -- but not, say, to birdsong or crickets. A zombie's attraction to human-generated sound is stronger than its attraction to other zombies, so if it hears gunfire, it will follow that sound to its source even if it later hears other zombies moaning in some other location.

Zombies are not intelligent and are not capable of abstract thought to any degree. Zombies cannot work door handles, open windows, etc. Instead, faced with an obstacle it cannot overcome, a zombie will beat and pound against it, trying to break through.

A zombie will never climb over an obstacle in such a way that its feet leave the ground and its weight is supported by its arms as it levers itself over. However, a zombie may fall over obstacles that are waist-high, if it reaches out too far and loses balance. Zombies can also walk up stairs, and across any other obstacle that they can physically step over.

A zombie will grab out for any human it sees and once it has ahold of a human, it will not let go easily. Because it feels no pain, its grip is very hard to break. A zombie will pursue a human being in any way it can. If its legs are broken, it will crawl. If its jaw is ripped off, it will still try to bite. And so on.

Zombies will not feed on a human for very long after the human is dead -- a couple minutes at most. They are attracted only to living people and "fresh" corpses. However, a mob of zombies can quickly strip a victim of its flesh, to the point that the victim doesn't even reanimate as a zombie, or, if he does reanimate, cannot move.

Zombies never attack one another. They do sometimes shove and push at each other while feeding, however.

Though undeterred by pain,  zombies still display a strong aversion to fire and water. A  person with an open torch can scare zombies back by waving it about, but this does not guarantee against being mobbed from behind or from aside. Zombies will also avoid treading into bodies of water much deeper than waist-high. That being said, a zombie cannot drown -- and if it does fall into deep water, it can survive at the bottom without any problems. A zombie will avoid fire and deep water as much as possible, even if their human targets escape beyond such obstacles.

Killing a zombie
A zombie can only be killed by destroying the brain. This is typically done in one of three ways.

1) Guns. This is the safest and surest way to kill zombies in large numbers. Of course, gunfire only draws even more of them, and only a headshot will do. Any caliber gun is capable of killing a zombie, and if you can hit one in the head, you're more likely than not to take it out with only one shot. However, as with humans, there are rare instances where a headshot is not fatal. This is very uncommon, obviously.

2) Blunt weapons. A zombie can be killed by cracking open its skull and causing massive, direct physical trauma to the brain inside. The force required to accomplish this is the same as it would be for attacking a normal human being with a blunt weapon. For that reason, this method is physically taxing and relatively inefficient -- in fact, dangerous -- for dealing with zombies in  large numbers. However, in small numbers, it is the preferred method. Zombies are easily defeated in one-on-one combat, and killing a zombie in this way is much quieter than using a gun, so it is unlikely to draw more.

3) Setting it on fire. A zombie will burn to death if set on fire. However, this process takes several hours -- and during this time, the zombie is a walking torch that presents a serious fire hazard. This is the least efficient and most dangerous way of dealing with zombies, and usually isn't done on purpose. A zombie that has been set on fire enters into a continuous panic mode, and will flail and stumble around quite a bit while it burns.

A group of scientists in our story are working on a nerve agent capable of killing zombies quickly, from afar, in large numbers.