I’ve been watching and for the most part enjoying “Walking Dead” on AMC. And like most folks that watch I have my own ideas about zombies.
In general my concepts could probably be assigned to any number of zombie stories. But I use Walking Dead as a basis because it’s current.
First off, how the outbreak began. It had to be either an air born or water born event.
It can’t have been just the recently dead reanimating. The relatively small number of bodies would have been easily contained. I imagine an event infecting a large part of the population like a plague. Rapidly killing them, leaving thousands to reanimate.
Now for the cause of the “virus”. A government accident is to easy and expected.
It’s an alien plot. Here’s how I see it. Aliens covertly disperse the zombie virus into the atmosphere. Thousands are turned into the walking dead. More are killed by the walkers. Survivors are left scattered and disorganized. After a year or so the aliens return, release an antidote. The walkers drop dead(again). With little resistance, there is nothing to stop them from raiding the planet for resources or even colonization. ( even as I right this it occurs to me the virus may even be an atmospheric prep for the aliens themselves )
Anyways that’s where my theory goes.
Thoughts from my head 
If you haven’t already I recommend you read the book World War Z: An Oral History of the Zombie War by Max Brooks. The concept of a few biters spreading such an illness is actually not unbelievable for a few reasons. At first nobody would believe it. There is nothing as powerful as misinformation when it comes to an event like this.
If you want to use the concept of a government conspiracy, there is the potential of a few knowing what it is and attempting to contain and study zombies in order to weaponize them for war. A government experiment gone wrong, or which they lost control of is therefore another method that would not involve water or air borne illnesses.
The problem with zombies coming about from a sickness in the air or water is two fold. It would wipe out everyone, unless some people are immune because there’s no escaping the need for air or water. And honestly if it was in the water then it could be contained some what by isolating the area infected.
I think what is key is people’s belief in what is happening. That is brought out in the book I recommended. Read it, it’s an incredible book and will really fuel your imagination.