Warning: here be spoilers
In many latter-day zombie movies, books, and TV shows, zombie-ism has a biological cause. In 28 Days Later, the infection is caused by the "Rage" virus, which escaped from a lab when animal rights activists break in and release a group of infected chimpanzees. Of course, one of the animals promptly bites one of its "liberators," and the infection spreads rapidly throughout Great Britain. In Zombieland, it's a mutated form of "mad cow" disease. The Crazies, it's the Trixie virus; World War Z, the Solanum virus; Resident Evil, the T virus. I could go on and on. Zombie causation has clearly evolved from the early days of radiation or curses, and has become a biological phenomenon in most modern zombie tales.
The Walking Dead is no exception. Though the claim is made in season 1, episode 6 ("TS-19") that the outbreak could be caused by just about anything--bacteria, virus, parasite, act of God--I call shenanigans. In the previous episode ("Wildfire"), Jenner, the CDC scientist, is processing tissue taken from Test Subject 19, and the visualization under his microscope looks very viral. Of course, take this with a few pounds of salt, since he's using a light microscope and can also see the nice alpha-helical DNA strains within the pathogen (in real life, things just don't look like this) and unless you're one of the giant viruses, you can't see viruses, much less DNA, under the microscope Jenner uses anyway. But still, it looks pretty viral-y to me, which is why I typically refer to it that way:
Microbial zombification makes sense in today's culture. My colleague Brooks Landon notes: "...zombies represent a better monster for the modern, post-9/11 world. They provide a release for feelings of being overwhelmed by abstract and intractable events like global economic crises, terrorism, and pandemics." In the past decade or so, we've seen the emergence of SARS, multiple outbreaks of influenza including a new pandemic strain, the continuing HIV crisis, Nipah, Hendra, more Ebola, just to name a handful. Infectious diseases are commonly in the news, and many times are unfortunately over-hyped, leading to a collective nervousness of all things microbial.
The infected zombie is further boosted by a number of recent studies, largely in insects, that demonstrate a type of pathogen-directed "mind control:" zombie ants, zombie grasshoppers, and zombie cockroaches, just to name a few. A recent video game has exploited the ant fungus idea, mutating it into a form that infects humans. Even rodents (and possibly humans) can have their behavior apparently influenced by a parasite called Toxoplasma gondii, which makes rodents lose their fear of cat scents and may influence the development of schizophrenia in humans, or more controversially, even affect sexual inhibitions. If germs are already controlling our minds--why couldn't they turn us into zombies?
And certainly, there are some candidate microbes which could, in theory, cause at least the "living" form of zombie-ism, even if they couldn't necessarily raise you from the dead. The Trixie virus, for example, is supposed to be a weaponized rhabdovirus--the family of viruses that includes rabies. Rabies virus infection certainly causes aggression and biting. The virus is spread via saliva, so biting is the main way it is transmitted between animals. In a recent book, Rabid, the authors trace rabies through history, and note that it may be at the root of many zombie (and vampire) tales. Rabies can also hide out in the body for awhile before showing symptoms, as the virus travels up the nerves toward the brain. This is why a bite near the head progresses to symptoms much faster than, say, one to the foot. Typical time from bite to symptoms is in the neighborhood of 6 weeks, depending on the location of the bite and dose of virus one receives, but extreme cases have been documented, with symptoms not showing up for as long as 8 years. And, like has been done on The Walking Dead, one of the ways that bitten victims would try to avoid symptoms would be to cut off the affected limb before the infection spread. (Ouch).
Could something like the "we're all infected" scenario used in the Walking Dead occur in real life? Maybe. With rabies, victims could appear physically fine for months to years. Even more extreme, there are a number of germs which can remain with people throughout their entire life. The virus that causes chicken pox, for example, doesn't ever really go away. Your body fights it off enough to keep it in check after the initial rash, but it hides out in your nerves and can come back in later years as shingles. Other herpes family viruses have a similar lifestyle: symptoms can come and go, but the virus never really leaves. The human papilloma virus (HPV) can also persist for years in some people (most infected people appear to clear this one, though). A bacterium called Helicobacter pylori can live very happily in a person's stomach--sometimes causing ulcers, but going completely undetected and causing no symptoms in most people. And of course, HIV, which does not go away except in a few notable and high-profile cases. So the concept is, as they say, biologically plausible.
The problem isn't necessarily with the microbiology, then, but with the epidemiology. How did everyone get infected so quickly? We know that the plague took an incredibly short time to spread (Jenner says less than 200 days in the first season, and "less than 63 days" since it went pandemic)--but how? That's a missing link in this scenario. We know the pathogen can certainly be spread by bites and then cause zombification that way, but other forms of inoculation (such as getting sprayed in the eyes or nose with zombie blood) don't seem to have that effect. Is it in the water? If so, that would be some damn rapid spread, since early on Jenner noted that this appeared to be a true pandemic--present around the world. How would that happen?
In the air? Possibly, but even most airborne microbes don't hang out indefinitely; they're dispersed by wind to levels below those able to cause infection, or killed by sunlight or other environmental conditions. So even if you had a herpes- or HIV-like virus that could hide out in the body for an extended period of time without causing symptoms, how did *everyone* get it in such a short timeframe? Some scenarios in other books and movies put the blame on bioterrorism. The above-mentioned Trixie virus, for example, was a bioweapon which was only accidentally released when the plane carrying it crashed. Spread of Trixie in the movie ended up being only local, but transmission beyond that is hinted at the end. A true bioterrorist attack could, theoretically, account for simultaneous outbreaks all over the world.
Finally, though the "infected zombie" is now the most common type, it should be noted that this isn't really new. George Romero, widely recognized as the grandfather of the modern zombie, acknowledges that he "ripped off" his idea for Night of the Living Dead from Richard Matheson's I am Legend--a vampire story from 1954. The cause of that vampirism?
Bacillus vampiris--a bacterium.
See also:
Part I: the microbiology of zombies
Part II: ineffective treatments and how not to survive the apocalypse
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Great article. However,
"We know the pathogen can certainly be spread by bites and then cause zombification that way, .."
I don't think we have evidence for that from the show. I think it clearer that zombie bites cause death, and there doesn't seem to be evidence that the agent that causes death also causes zombieism( or vice versa). In Walking dead, any death is a sufficient condition for becoming a zombie. I would guess that zombies cause death because of a massive polymicrobial infection/sepsis.
Interesting that zombies have become the predominant horror movie "monsters" of our era, and that this seems to have some correlation with post-9/11 culture shifts.
The zombie archetype also provides a kind of emotional narrative or "explanation" for how a handful of hateful men could have had the motivation to fly airliners into buildings and into the ground, and for how the "meme virus" of Al Qaeda spreads itself to new recruits. Thinking of terrorists as zombies, and using zombies as mythic stand-ins for terrorists, is certainly better than going down the trail of racism and bigotry as might have happened in an earlier era.
Bioterrorism has replaced nuclear war as the sudden/unpredictable existential threat of our times. Everyone knows, at some level, that all it takes is another handful of hateful men to cook up a plague, or alternately, pick up one somewhere (e.g. Ebola) and then spread it around enough to cause mass panic and other consequences.
Lately I've been trying to wrap my brain around the question of why people deliberately like to expose themselves to emotionally unpleasant media, from sad songs to violent films, including "splattery" zombie films. Seems to me that one potentially viable explanation is that the sensation of the unpleasant emotion wearing off after the performance, and the normal emotional homeostasis returning, is as inherently pleasurable and reinforcing as a shift from a normal mood to a pleasant mood. Both are cases of "movement toward relative pleasure", and could be translated to neurochemical mechanisms and tested. The first place I'd look is at comparative levels of adrenal hormones and endorphins, before, during, and after exposure to unpleasant media.
As for cognitive/behavioral effects of transmissible pathogens on humans, it would not surprise me if these were far more common than we expect.
For a very well researched variation on the viral-zombie-plague meme, may I recommend John Ringo's Under a Graveyard Sky (ISBN 1-4516-3919-8)