What is the cause of the zombies in "Walking Dead"?

In the comic book, what causing everyone to turn into Zombies. Do they state it. If so, what was it and which comic is reveals it?What is the cause of the zombies in "Walking Dead"?
No, it has not said yet but it well very soon, right now the comic series is on book 77 and it comes out monthly.What is the cause of the zombies in "Walking Dead"?
So far there hasn't been an explanation for it in the show and although I don't read the comics from everything I've read about it and everything else about the show I've not heard of the cause yet.

So far as I know they've never revealed it.

That's not all bad,adds a little more mystery to the show/comic.

Any theories you've come up with?

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