I'm wondering where and why the zombie outbreak began?
The cause of the outbreak is not known. It has been made fairly clear that the initial source of the virus is not currently something that either the comic or the show are planned to address, as this quote from an interview with show producer Glen Mazzara indicates:
Will determining the cause of the outbreak be something that the group, now that they know they're all infected, spends time on?
Mazzara: Robert has not been interested in addressing in the comic book, and I'm not interested in addressing in the show. That being said, if it leads to new story -- if there's something that's important that we get out of it -- I'll be the first one to write it. But right now the cause of the zombie outbreak seems irrelevant. I always want the show to play like a horror movie every week. If you define what caused the outbreak, that puts us in a world of science fiction, and this isn't science fiction to me, it's horror. In my mind that's two different genres, so that is an important distinction to me.
All we really know about the virus is:
Everyone has it
It remains dormant in the brain
Sometime (anywhere from a few minutes to several hours) after the moment of death, the virus activates, and reanimates the body
We can also speculate that humans are likely the only animals that the virus infects.
All we can really infer from this is that the virus is likely airborne, due to how widespread the infection is.
"No explanation is offered for this behavior -- indeed, what explanation would suffice?" -- Roger Ebert
Logically there must have been a start to it, a Patient Zero, but that start wasn't revealed to us. The characters don't know how it started. Even Dr. Jenner of the CDC, who watched the progression of the contagion from the first reports, and had every possible research tool at his disposal, had no idea what it was and didn't seem to know where it really began.
This follows the real-life pattern of diseases like AIDS and Ebola. We can study them right down to their molecules, we can trace them back to a few square miles in Africa, but that might be as far as we ever get.
Not sure if it helps, the io9 article Everything you need to know about The Walking Dead explain why it has never been revealed.
What causes the zombie uprising in The Walking Dead?
In the comic, it's not entirely clear, and it's kind of a moot point anyway. Kirkman's said that he'll never reveal the origins of the undead uprising. Furthermore, it's just not the series' foremost concern. The main characters aren't super-sexy undead researchers. They're average schmos with average lives, and — barring one or two seemingly unkillable characters — they die extremely easily. The series is about staying alive (i.e. finding food, ammunition, and shelter; not going insane). We have yet to see whether the TV show will address the zombies' origins.
TL;DR: We don't know, and according to franchise creator Robert Kirkman, who writes the comics and is an Executive Producer on the show, we never will.
Why We Don't Know - In-Universe:
They Wouldn't Know:
There are no newspapers anymore. No TV. No internet. No telephones. No radios. Information can only travel by word of mouth in the post-apocalyptic world. The people best suited to discover how and why the outbreak began - doctors and epidemiologists - were among the first to die. When the outbreak began, people who had been bitten went to the hospitals; then they died, turned, and quickly overran the hospitals. Doctors and nurses were in the greatest danger, and few survived the initial stages of the epidemic.
Epidemiologists like Jenner at the CDC were safer than hospital staff, but some were bitten, some lost hope and killed themselves, and as the world outside fell apart, many went home to be with their families. Soon, even those who kept working were unable to continue, because the infrastructure began to fail - they lost contact with the outside world, the water stopped running, and eventually, the power went out for good. As Jenner said, the scientists just didn't have enough time to figure out what was causing the problem, let alone how to fix it. And even what little they did know was impossible to share with anyone outside their own facilities, because the means of communication were breaking down.
It all happened so fast that the scientists trying to find a solution to the problem didn't have time to establish what was causing it, let alone share their findings with the public.
Realism - except for the existence of zombies, obviously - is important to Kirkman:
Kirkman wants TWD to feel real. The closest the TV show came to explaining the outbreak was the last episode of season 1, when the group went to the CDC and got a tiny bit of information. This actually never happened in the comics, and this is one of many cases where the comics beat the show. The CDC episode seems unrealistic and artificial, because the odds of a random group of people getting there, being allowed in, and finding the last man standing is ridiculously small.
Kirkman himself has said the CDC episode was by far his biggest regret regarding the show:
If I had to do it again, I wouldn't have done the CDC episode. It possibly gave away too much information and was such a big change very early on in the series... I've been careful in the comic series to not say what's happening in other parts of the world... The fact that France is mentioned in that episode and other things like that, I probably would have steered away from that stuff if I had to do it all over again.
Q: Why did you not like the mention about France at the CDC in season 1?
A: I feel it revealed too much of the world and gave the characters too much information. I prefer the way they're more in the dark about the rest of the world in the comic. For instance, for all they know in the comic, the outbreak is contained on this continent.
- Reddit AMA with Robert Kirkman
So they know almost nothing on the show, and even less in the comics - and Kirkman prefers the latter.
It Wouldn't Matter Anyway:
How would knowing how and why the outbreak began help people like Rick's group? It wouldn't. All they need to know is that the world has ended, strangers can't be trusted, walking corpses are everywhere trying to eat people, and you can kill them by destroying the brain. That would be true whether the outbreak was caused by a virus, or bacteria, or a fungus, or radiation, or magic, or aliens, or an act of god. They might wonder about it in their rare moments of peace and safety, but having the answer wouldn't change anything. They are desperate people fighting to survive a living hell.
People in The Walking Dead aren't obsessed with finding out how the outbreak began, for the same reason people floating around on the ocean after the Titanic went down didn't wonder which glacier the iceberg that sank the ship came from, how big it was, or when it calved. In both cases, there were much more important things to worry about.
If You Were In Their Shoes, You Wouldn't Know Or Care Either:
Picture yourself in the group's situation:
You hear weird stories about people biting each other. Then you see it happen. Someone says the biters are actually dead p̤eople, which is too stupid to believe. Then someone you know is bitten, dies, and attacks you. The government tells you to stay home. Two days have passed. The next day, you phone goes dead. The government tells you to get to a big city. You try, but you're sitting in traffic when the government broadcasts stop; you see planes dropping napalm on the city ahead. You go back home. A week has gone by. The power gets shut off. No news of any kind is available now. Someone starts pounding on your door. You hear gunfire nearby. The shooting stops. You hear moans, then screaming, then more moaning.
You look out the window - three zombies are eating your neighbor. You bolt out the door, hop in your car, and head for the countryside. The few radio stations that are still broadcasting are all playing a looped message telling people to lock their doors and wait for military assistance. Every town you pass through looks worse than the last - burning buildings, corpses in the streets, roads blocked by abandoned and crashed vehicles, looters, zombies, fire, blood, death - the world is falling apart before your eyes.
You reach a campsite with people who seem to be decent enough, and fight off frequent zombie attacks together. No one knows more than you do - there are different variations of the same rumors about safe places far away, but deep down, you know they are all wishful thinking - nowhere is safe anymore.
So, now that you've imagined all of this, what part of the story makes you think you'd know how, why, and where the outbreak began? What part of the story makes you think you'd even care, when you're so busy worrying about when the next attack will happen, how you're going to get food and water, when a herd too big to fight off is going to show up, how long it will take for the people in your group to turn on each other, and when armed raiders will storm your camp, rape and murder everyone in it, and steal everything in sight?
This was Kirkman's reasoning when he decided that we shouldn't learn anything specific about the origins of the outbreak.
Word of God - Why We Don't Know, Out-Of-Universe:
Franchise creator Robert Kirkman, who writes the comics and is an Executive Producer on the show:
As to the cause of the zombie outbreak, Kirkman wrote, "I have ideas [about the cause of the zombie plague]...but it's nothing set in stone because I never plan on writing it. So yes...I do know...kind of."
In response to a question:
"I think you should elaborate more on... how this whole mess started in the first place. Was it like a plague or a rapture kind of thing?"
"...That starts to get into the origin of all this stuff, and I think that's unimportant to the series itself. There will be smaller answers as things progress... but never will we see the whole picture."
[Revealing what caused the outbreak is] not the priority in Walking Dead. It's not the priority in Fear the Walking Dead. It's not about fixing this world, and it's not about learning what the cause is. I know the origins of the zombie apocalypse, I just think it's boring to explain it."
Q: How much will you reveal about the origin of the outbreak [in Fear the Walking Dead]?
A: Next to nothing. I still maintain that's not an important aspect of the story. I think watching "Fear the Walking Dead," you'll get a bigger picture of the world and there will currently be aspects of watching civilization crumble. It will give you a better insight into what is happening here. As far as digging down to actually find the smoking gun and realizing what that causes, it's really just unimportant to the overall story. You'll see when you start watching the show. If we were to do a spinoff of "The Walking Dead" and it was about a bunch of scientists who were working to find the cure and finding out the origin, that would bore me to tears. I don't really know where that show goes.
I don't find [the question of how the outbreak began] to be interesting at all. In zombie fiction, you see the outbreak, you see the first days and the craziness in almost every other story told in this realm. I'm very proud of the fact that, for the most part, The Walking Dead has skipped over that part. Go watch Zach Snyder's Dawn of the Dead movie to see the early days of this, or Shaun of the Dead does it really well.
The Walking Dead Showrunner and Executive Producer Scott Gimple:
Q. From a narrative standpoint, what is the value of withholding the source of the plague?
A. This is definitely something I’ve followed Robert’s lead on, and I really have come to value it a great deal. This story isn’t about that. Hamlet is handling the plague stuff. We’re telling an epic tale with Rosencrantz and Guildenstern.
Former Showrunner and Executive Producer of The Walking Dead Glen Mazzara:
Robert has not been interested in addressing in the comic book, and I'm not interested in addressing in the show... [R]ight now the cause of the zombie outbreak seems irrelevant. I always want the show to play like a horror movie every week. If you define what caused the outbreak, that puts us in a world of science fiction, and this isn't science fiction to me, it's horror. In my mind that's two different genres, so that is an important distinction to me.
Fear the Walking Dead Showrunner and Executive Producer Dave Erickson:
Q: Kirkman has said the origin of the outbreak is something he's never going to reveal in the comics or the flagship series. Is that true here, too?
A: I had a couple of early pitches that touched on what you're referring to and Robert shut me down. For him, it's never been about what caused it; it's always been about the impact it has on people.
Fear the Walking Dead Executive Producer David Apert:
Don’t hold your breath, though, waiting to learn what turned people into zombies.
"We’re going to get a lot of theories,” Alpert said. “But for us, why this thing started and the origin of it is not important. It’s never been about zombies and where they came from, it’s about the people. It’s about our characters and how they deal with it."
If this were really happening, it is totally plausible that no one would know where or how it began.
We haven't been told how it all started, and chances are, we never will be. Kirkman dislikes it when zombie media explains too much, and has been careful to avoid following the same path as his predecessors. The closest any The Walking Dead format has come to explaining the origins of the outbreak was in the first season finale of the original show, and - not coincidentally - Kirkman calls that episode his biggest regret, vis a vis the TWD franchise.
In the webisodes a terrorist attack was mentioned. At the end of series 2 it was revealed that everyone is infected and they will reanimate after death. There would have been a further explanation to the outbreak in a planned webisode. Remember, in series 1 when Rick jumps in the tank and there is a walker in there? Well this was to be the ending of the planned webisode. The walker was a US Army Ranger in Atlanta trying to evacuate the population during the outbreak but the city was overrun. This webisode was shelved due to budgeting issues. It is a shame because it would have given us a picture of what happened prior to Rick waking from his coma at the hospital.
In the comics, and I am just assuming the show, it is an infection. It is not believed to be air-born but basically everyone has it. You fall down stairs and break your neck, you're a zombie.
Quote from Robert Kirkman:
The rule is: WHATEVER it is that causes the zombies, is something everyone already has. If you stub your toe, get an infection and die, you turn into a zombie, UNLESS your brain is damaged. If someone shoots you in the head and you die, you're dead. A zombie bite kills you because of infection, or blood loss, not because of the zombie "virus."
He has also said he won't reveal the cause in the comics so we will have to wait to see on the TV show but I wouldn't hold your breath. I guess it is up to the viewers own imagination.
There can be many explanations for the outbreak of the infection:
It broke out as a common virus and became a global epidemic in the few months that Rick was in coma for. As in the first season The CDC was working on it, this shows maybe that it started as a virus.
It started off as a terrorist attack as in the Comics.
It started off like in "Resident Evil" i.e. by mistake maybe, there is no proof for this, but it is just a possibility.
In the TV series, they differ greatly from the comic book series, in the TV series the infection is more closely and probably intentionally styled after the Solanum Virus from "The Zombie Survival Guide" and "World War Z". The difference being is that with the Solanum Virus or with the Walking Dead, everyone is already infected. But bites cause a lethal infection that kills you, and then you are reanimated. How this started is a little sketchy, I imagine the virus that everyone was already infected with mutated and when someone died they came back.. this is probably why infections started out in multiple locations and there is no true "patient zero"