Something interesting is going on neurologically, metabolically and biologically when the recently deceased become capable of getting up and trying to kill the living. Firstly, any pathological study must start with an examination of methods of propagation, so that the disease can be contained. Secondly, we look at the role of the cortex of the cerebrum in zombies and animals. A broken cortex can possibly result in more zombie-like behaviour, so, our description of zombie biology must account for the lack of a working cerebral cortex. Likewise, the limbic system that controls our motivations and drives must be strongly altered. Thirdly, zombie bodies are special. They do not feel pain, eat or drink. Normal humans die of thirst quickly. No zombie film has explained, really or fully, how the infection causes all of these unlikely symptoms. But we shall try...
The study of our nervous system, particularly of the brain.
A Virus. By far the most natural-seeming cause of a disease is a virus. Perhaps human-created, or perhaps the result of a natural freak. This virus infects the blood and saliva... so that getting bitten, like rabies, causes infection. The virus attacks the body and eventually is fatal, at which point braindeath appears to stimulate the virus into a second phase: It alters brain activity in a precise way, taking control, and therefore somehow causing the dead to rise, seemingly alive again. No zombie film has depicted a cure or vaccination against such a disease, and no people have been found to be naturally immune.
Generic Science Paranoia 1: 'Radiation' causes the disease. In Night of the Living Dead radiation from a space probe that returned to Earth may be the cause of the epidemic. All the recently deceased return to life. There is no need for this to 'spread', it is simply universal. As our brainwaves can be measured in hertz, and parts of the human body are reactive to different frequencies of radiation, it stands to reason that some radiation can affect our brains in specific ways. This very specific radiation must alter a tiny part of the brain, which is not important enough to affect a living person. Perhaps some primitive part of the brain that is overridden by the cortex. When the person dies, the part of the brain modified by the radiation becomes the centre of brain activity, thus determining the pattern of brain activity. This primitive behaviour involves the person trying to eat, or kill, the living.
Generic Science Paranoia 2: A man-made substance or virus causes the outbreak. It is often poisonous and fatal, killing those who come into contact with it. It behaves like a virus, so that infected people generate more of the substance, and therefore it spreads. The rest is similar to Method 1 above.
We know that the most complex behaviours in highly developed animals such as mankind are associated with the cortex, the outermost layer of the two hemispheres of the brain. If you remove the cortex in animals, their behaviour becomes purely instinctive and 'stereotyped'. They can repeat over-learned behaviours and have instinctive drives. In other words, they become a bit like zombies.
“The cerebral hemispheres are the two largest structures at the top of the brain which enfold (and, therefore, conceal from view) most other brain structures. [...] The top layer of the cerebrum (about 1cm at its deepest) is the cerebral cortex (usually just called 'cortex' which means 'bark'). [...] About three-quarters of the cortex does not have an obvious sensory or motor function and is known as the association cortex; this is where the 'higher mental functions' (cognition) - thinking, reasoning, learning, etc. - probably 'occur'. [...] There is no doubt that the cortex is not necessary for biological survival [...] as some species do not have one to begin with (e.g. birds) and in those that do, surgical removal does not prevent the animal from displaying a wide range of behaviour, although it becomes much more automatic and stereotyped.”
In human zombies a form of incomplete brain death must occur. It disables the higher functions of the brain and leaves behind the mechanical and motor controls of the limbic system, complete with a small amount of memory function. This results in the limited aspects of zombie behaviour; they can remember how to move around and other motor skills (such as eating), but lack coherent consciousness or higher functions.
Some zombie films have it that willpower does not come from the brain. I have called these the animist zombie films on my zombies!!! introduction page. These depict zombies in an (even) less likely light. Motor functions in our nervous system are controlled centrally by organs at the base of our brain, at the top of the spine, such as the medulla and cerebellum. Orders travel from here, down the spine, to the appendages. The appendages' nerves have no system to self-rule. In other words, muscles can only be controlled in a directed way, from the brain. As a result of this, animist zombie films must somehow explain that (for example), a hand on its own has had its nerves physically rewired into a new motor control area. It becomes far too complicated and unlikely that mystical sources of willpower could ever accomplish this.
Zombie bodies are human bodies, but with changed properties. Normal nervous system activity seems lacking, and only motor control exists. They do not feel pain, or at least, do not react to it. They do not seem to suffer much from blood loss and do not need to eat or drink. You could guess that they have no internal homeostasis; they retain most their water because they do not sweat, which means they do not regulate body temperature, but also they are more water efficient. A person can survive without food for weeks, but, without the bulk of their brain functioning the energy requirements of a zombie is lower by up to 10%. They can survive longer without food. In 28 Days Later the zombies become skinny and begin to die from starvation. Muscles use energy and require blood sugar to function. The blood must transport acids and the by-products of metabolism away from the muscles and organs to be disposed of or recycled. Without a beating heart, our muscles and bodies would become immobile. With no diet (apart from occasional fresh meat, which most zombies do not get), they must be burning their own body fats and even muscle proteins... yet their continued ability to move contradicts this. In some more realistic films, zombies can be killed like normal people: Damage to their body will stop them. The biology of the zombie body is the weakest-link in this genre's scientific credibility.
Zombies can always tell who else is a zombie, apart from in a few of the more humorous zombie films (in Shaun of the Dead the detection is purely visual so that with a bit of zombie makeup and some fake moaning, survivors can sneak past the undead). In the vast majority of zombie films, the undead do not fight each or eat each other, and mingle peacefully. They only attack the living. Some chemical interaction must occur: The dead can either smell who is dead, or, can smell who is alive.
If the dead can smell each other, and therefore know who to ignore, some sort of zombie pheromone must be released by the dead. If the cause of the disease is a virus, perhaps by-products of the virus' actions are detectable. Perhaps this by-product is unpleasant: If the zombies need to eat fresh meat, then decaying meat may contain poisons that they can detect.
Or perhaps it is the living who produce normal pheromones, and the dead can merely smell these (and they smell like food). This would however imply that sometimes the living could be smelled out by the dead, whereas in zombie films this never happens. In 28 Days Later one character hides behind a mirror and in a kitchen cupboard. The dead would surely be able to smell them clearly, and find them.
In both cases, it is strange that in crowded rooms full of the dead and the living fighting one another, the dead never accidentally attack each other. Even fully conscious, thinking and functioning living Humans do it, why don't zombies who have largely impaired brain function sometimes attack the wrong target? Point (1) above looks more likely, but perhaps both points play a role: The zombies can both smell the unpleasant flesh of other zombies, but also smell the fresh flesh of living humans.
In the World War Z film, a central part of the plot becomes the fact that the zombies select viable new victims and not those who are mortally ill from disease. Infection with some diseases confuse the zombies, and make them overlook certain human beings. An early comment in the film says that about 5% of the population are seemingly immune - although no such people are discovered in the film, it appears that this initial report was referring unknowingly to those have terminal illnesses that make the zombies avoid them. They flow around them like "a river around a rock" to quote from the film. just in the same way that they might flow around their follow zombies. In an unbelievably short few hours after Brad Pitt injects himself with a horrible (unspecified) virus, he is emitting a pheromone that makes the zombies ignore him.
The problem is, of course, that aside from the smell of rotting flesh, there is no such chemical aroma that indicates our general health in such a specific way, and in such strength that our unaided noses can detect it. Most horrible and fatal diseases do not cause any particular smell. The zombie virus enhances our olfactory system in addition to manipulating the brain. As these necessary properties of the virus builds up, it starts to sound increasingly impossible. I know, of course, that very few who watch zombie films care about the biological plausibility of the phenomenon, but, just take it for granted that World War Z's description of the outbreak is about as advanced as any zombie film has gotten so far, specifically for its attempt to describe how zombies can 'detect' each other and/or 'detect' the living.