God I'm getting tired of the "they can turn any moment even if they're still alive" and the "Rick is special/immune" crap.
Jesus Christ this.
TWD is based on the traditional zombie lore inspired by the George Romero movies. In which anyone who dies for any reason reanimates as a zombie. Being bitten causes death... presumably due to the amount of bacteria etc... This death leads to reanimation as a zombie.
Other than that simple insignificant background rule this is a character based drama. Special immunity, or zombies turning into Lickers or Rick turning into a Nemesis complete with mini gun arms will not benefit the story in any way.
The show could be set in any post apocalyptic world and wouldn't be hugely different week to week, pretty easy to replace the zombies with raiders etc and your story doesn't massively change. They've decided to do this story in the Romero zombie universe, they've even gone to the extent of ensuring that no zombie moves faster than the fastest zombie in NotLD, as they love and respect this fictional universe- there is literally no incentive to start changing it.