Let's Talk The Walking Dead - Spoilers!!

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The show first hinted at this possibility a couple of weeks back when Rick and Shane made the first trip to release Randall and encountered two security guard zombies that showed no signs of having been bitten.

OK, but why were they "dead?"

If I was one of the good-for-nothing writers on this show I'd just explain all the inconsistencies by playing the "mutating virus" card.