Not only a scratch, but one of the disappointing aspects of the show is how when they go hand-to-hand, they don’t do anything to protect themselves from blood/brain splatter. Any infected zombie DNA gets in your mouth or a cut, you’re dead. In the first few episodes it showed them wearing protective clothing and a face mask. Guess it was too much money or it hid the actors’ faces too much.
They are already infected, I think if they get a life dangering injury they become undead...
Jim was bit in Season 1 and died of "the fever" associated with the infection. The wound itself was not life threatening.