We'll probably never know what exactly caused the initial outbreak that led to the walkers in The Walking Dead - after all, the show isn't really about finding those answers.
However, producer Gale Ann Hurd has come up with her own suggestion, and it's not as outlandish as you'd think.
Asked during a Fear the Walking Dead Comic-Con panel about what caused the virus in the first place, Hurd answered without hesitation: "The meth from Breaking Bad, for sure."
Comic creator Robert Kirkman added: "That's canon, it's confirmed!"
OK, so we don't think it's actually canon, though there is weight to the idea, especially as there is a long-running fan theory that The Walking Dead is set in the same universe as Breaking Bad.
Last year, for example, Fear the Walking Dead delivered a possible Easter egg in the form of song 'Negro y Azul: The Ballad of Heisenberg' from Breaking Bad.
"I tried to be subtle with it," said showrunner Dave Erickson at the time. "That was probably the moment when I fell the most deeply in love with Breaking Bad, when they did that cold open music video.
"As we were looking for pieces to incorporate into this world, it had the right vibe to it. It's a gentle nod of admiration and adoration to [Breaking Bad creator] Vince Gilligan."
Also, back in season one of The Walking Dead, Merle Dixon seems to be in possession of a hidden stash of blue-sky meth, much like that in Breaking Bad.
In short, we may have Walter White to thank for the zombie outbreak. Thanks a lot, Heisenberg!
The Walking Dead returns on October 7 on AMC in the US, and the following day (October 8) on FOX and NOW TV in the UK.
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