Vandy, I can answer that one for you- We're actually all supposed to be immune, excepting a few rarities, because at this point in the zombie-land-times, most of the people still alive -are- the ones who were immune, or in really large groups.

Five to eight percent of humanity is still a lot of people, by zombie apocalypse standards. So we're more or less just the people that -can- survive getting bitten. Most of the characters probably have been at some point.