"The Walking Dead" is full of mysteries, and for most of the series's existence, the origin of the zombie virus was the biggest mystery of all. Rick Grimes (Andrew Lincoln) and all the other series mainstays learn plenty about the walkers that have overrun the planet, but they never do figure out where the zombies came from in the first place.

The "Walking Dead" spinoffs helped give us a global view of the zombie apocalypse, but only one revealed the biggest secret in the franchise. A post-credits scene in the Season 2 finale of "The Walking Dead: The World Beyond" takes viewers to a biomedical lab in France, where a woman is desperately rooting around in dusty cabinets for computer drives. After watching an old video from Dr. Jenner (Noah Emmerich) at the Centers for Disease Control, she

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