The walking dead media shows that whatever it is, everyone has it - so long as you don't die from brain trauma, you reanimate, so everyone "carries" the virus that doesn't start until death occurs. The bite just kills you.
But whilst this works for Romero's Dead trilogy, it's retarded in TWD, as it's shown to be an actual virus that the CDC can pinpoint and show which leaves more questions than it answers. The bite even from a completely fresh reanimated zombie still turns you (so it isn't bacteria/infection), and humans who bite other humans don't turn each other.
At some points in both the comics and the TV shows and probably even the telltale games, they rub zombie guts into their faces, eyes and mouths to blend in, and again, they don't seem to be affected at all. In the show they coat bullets in walker blood to infect people who survive firefights later on, so getting flesh in open wounds turns you supposedly. Or it doesn't. It's inconsistent shit. and they should have left it intentionally very vague.