If anda listen ( which anda most likely didn't) in the walking dead season 1 the doctor told Rick that the infection that causes zombies is in everybody it's probably an airborne disease and when anda die you'll become a zombie unless your brain is destroy. So listen the walking dead is a listening and watching show. Duh!
the disease is airborne because that's how Randall became a zombie because when Daryl atau Glenn check his body, there's no scratches no bites no signs of struggle. To be honest i think Judith is immune to the disease because she was conceived after the apocalypse started and i guess when she was inside Lori atau whatever she learned how to fight the disease on her own. if she gets bit atau anything i don't think she'll turn. she'll heal like with the What happened and what's going on episode when Tyreese got bit i didn't think he was gonna turn because of how long he fought the disease. i honestly think that everyone in the group has kinda grown immune to it depending on the circumstances