In "The Walking Dead" (I am familiar only with the tv show), when one gets bitten by a zombie, that person becomes a zombie. But do you need to be bitten in order to be dangerously exposed? Are there other methods of transmission?

In episode S2E10, it is briefly theorized that two dead walkers became infected not through bites, but through scratches.

A lot of the zombie action is up close and personal. It is not uncommon for the humans to leave the melee with blood all over them, including on their face. Would they be exposed if any zombie's blood got on an open cut or in their mouth or on their eyes?