How do you become a zombie on The Walking Dead?

I’ve just finished all the seasons 1-2 of The Walking Dead DVD and I wonder that in the Walking Dead, when Shane is killed he comes back like 5 seconds later as a zombie. Don't you have to be bit to become a zombie?

A5: In the show, everyone is infected because the virus is airborne. I think that's what Dr.Jenner Whispered to Rick in Season1 Episode 6! But if your shot in the head, you will not turn!

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