That's a staple of Romero Zombie-lore. It's not something that they just "came up with."I guess it just bothers me because it's different from every other zombie story. In every other zombie story, you're OK until you get bit. Me saying it's a plot hole is probably a little unfair. It's not a hole until the story is finished, and we don't know where the creators will take us.
This show/story is different from every other zombie story ever. They threw this plot point out there that everyone is already infected. There's a reason they changed up the rules of zombie mythology, right? They wouldn't just change it up for no reason.
Maybe they've already used that plot device to scare the viewer into realizing that no one is ever safe even when the zombies are locked away. Even the people of Woodbury aren't safe because anyone can become a zombie from a heart attack or a fall down a flight of stairs. BUT, I'm hoping there's more to it than that.