Its viral, its been confirmed as such in the tv show, and the comics. Origin doesnt matter, since no cure will be found, ever.

Diabeetus caused it.

A cure is completely irrelevant. The point of the show is that they're not going to be cured; they have to deal with it.

Realistically, a zombie apocalypse would be like a month long.

Necromancy. [spoiler]In all seriousness I would like to see a modern Zombie Apoc that was caused by necromancy, it would be a welcome change from all the virus and pathogen shit now days. [REC] was pretty close, although it's demonic possession not necromancy.[/spoiler]

28 Days Later is actually a prequel to The Walking Dead.

(Only based off the TV series, so pardon any missing info that debunks this) It would have to be some sort of airborne virus if it were to be worldwide within the time Rick was in a coma. It had to be a dormant or trigger-activated thing for it to cause so much chaos quickly because... [spoiler]Doctors would notice that people dying from natural causes/terminal illnesses/accidents were reanimating, and precautions such as quarantining near-death people and news alerts would be out before it became uncontrollable. The part of the virus which makes dead people just rise up would had to have activated all at the same time, or there would be more information available. Even if it was still uncontrollable, you'd think some survivors other than Dr. Jennings in the CDC lab would know earlier...[/spoiler] It also would have to have some lethal component in it, as a standard bite from a zombie would do nothing (other than hurt like hell, and maybe some sort of infection due to rotting. Noticeably the zombies aim for the neck on unaware victims--->quicker to kill (and stealthy, in some cases)--->faster to reanimate) As for how...scientists effed up a cure for something...or it's a conspiracy! It was planned all along, given the possible dormancy of the infection.

They might die off eventually. Don't zombies just go dormant until they are alerted that there is something alive to feed on?

Some parasitic form of mutant rabies, even though it's supposed to be scientifically impossible.

It probably mutated, if its airborne then there could've been an accident while finding a cure. Which means there could still be some remnants of civilization left. I mean one small town survived, how many other towns survived and possibly banded together? A result could be some surviving scientists trying to find a cure and accidentally releasing a mutation.

[spoiler]it is a virus. everyone is already infected and a carrier. it takes death to make someone turn. bites cause sepsis and massive bacterial infection due to the fact zombies are decaying flesh with all kinds of decomposing bacteria [think komodo dragon/monitor lizard spit], and even a normal mouth has many kinds of bacteria.. [/spoiler] well if you haven't read the comics, i suggest getting the compendiums and novellas...spoilered for those who haven't.

I just think the fact that everyone is infected and that France was near completing a vaccine/ cure for it before their reactors shut down is pretty interesting.

It was probably some strain of the rabies virus. That at least would make the most sense. Maybe it stays dormant in your body until you die? idk