TIMSTA_THE_PIMPSTA : There are a few possibilities I can think of; one is that rabies, a disease that is most commonly related to the symptoms of a zombie, could evolve (or be bio-engineered as a weapon) into a super disease. This disease could easily infect anyone, and even if the cure for rabies could work, the process is already long and expensive. This makes it impossible for a large outbreak.
Another reason is that stupid ass Scientist developing biological weapons for wars could horrifically destroy humanity. Transversely they could be developing super human injections for war that would allow soldiers super human strength and higher pain tolerance. This could easily go awry due to the affect being permanent. Also, any such genome-therapy would be based in the nervous system, allowing easily a virus-like spread through bites. If such a development were real, it could easily over-infect the victim leading to a complete breakdown of reasoning, leading to essentially a zombie outbreak.


Here is Richard Matheson's (the man who conceived our entire concept of the modern-day zombie) take on how the Zombie Virus might have evolved: The virus evolved from a mutation brought about by the Eastern United States being nuked, and carried by the ensuing dust storms. The germ would be a bacillus, a cylindrical bacteria, that creates an isotonic solution in the blood and circulates the blood slower than normal, still maintaining all bodily functions. In order to produce energy, it has to react with the living cells in fresh human blood thus explaining Zack's insatiable lust to kill. When deprived of blood, it makes self-killing bacteriophage's (proteins that destroy the cells of the host body, causing the zombie to rapidly decompose) or else sporulates (goes into hibernation, killing the zombie and moving on to find another host). Seriously, read the book.
--TonyPro
You wouldn't have to release it in a large area. A small area, 1 infected person can do a lot of damage plus the fact that todays generation are quite stupid. Most people are just too busy to go to hospital to get a check up, "can't lose a day at work" they'd say. Before you know where you are you're knee deep in the dead and royally screwed. City-folk Pah!!. Thinking aloud Farmboy74

I always like to think of a bio-weapon escaping from the laboratory, like Resident Evil. Either that or a horrid mutation of a virus like the bird flu. from Vanrulzz
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Alicestar:
Well if were looking at a mind controlling virus, two distinct ones come to mind. mad cow, and rabies. although rabies doesn't control is subject it just juices your system with rage inducing hormones, and mad cow just eats holes in the neural pathways so all were looking at there is heavy brain damage. were looking more toward resurrecting dead bodies so I'm thinking more along the lines of a parasite. With a parasite you will find that it is in itself a living, intelligent organism. some parasites have been known to control the behavior of there host. also parasites are extremely resistant to medication. now if a parasite was to surface that could control a host (living or dead) that used human saliva to pass its offspring (eggs) and also used human blood or tissue to sustain itself then we would have a full fledged zombie on our hands wouldn't we? I Know this parasite sounds highly unlikely to you but it might be that such a parasite actually exists, if you think about it a vampire is nothing more than an intelligent, glorified ZOMBIE, just think about it.

Ciper234: It would be possible to alter current diseases to get one like although it could vary; it could be like the rage virus. also there could be a dawn of the dead thing where radiation revives dead tissue and only brings back primal instincts.

ZEROFLY: All I have to say in this one is either a man-made virus, or something that is created in nature. Those are basically the only two options. If all hell breaks loose, and it doesn't look like Bambie did it, blame the government.

Roxas33: I say government experiment gone wrong or a bio-weapon or something of this sort.

UnUndead: I think someone obsessed with zombies will try to find a cure for the so far non-existent Z-virus, and end up injecting him or herself with a Z-virus that they created. Just because that's the most ironic cause I can think of.
See also:

• Deteriorating tissue impacts
• First Signs of a zombie bite
• First signs of a zombie outbreak
  
• Why hunt the living?
• Motor Skills
• Nonhuman Infection
• Possibility of "recovery"
• Psychological effects on the living
• Experimentations
  
• Origin of the Zombie
• How are zombies created?
• Celestial Events

Ally_Zombie26
The truth is, the Resident Evil scenario, one city (of whatever size) is likely to be the greatest extent of the outbreak; As one city falls (let us call it Raccoon), the word will very quickly spread as newscasters there begin sending panicked reports abroad, those who were in Raccoon within the last few days would be quickly quarantined to see if they, too, are infected; Flights, trains, and busses coming out of there will be sequestered, those heading in diverted (possibly by National Guards).
Additionally, Law Enforcement and EMS abroad will quickly realize the nature of the threat, and take the measures necessary. The absolute, total, God-help-us-we’re-all-gonna-die, really DOTD scenario might, maybe, cover an entire state. Far more realistically, it would engulf a full county (even this is pretty apocalyptic).