how can a person like shane turn into a zombie without being scratched or bitten

WARNING SPOILER ALERT!!!!!!!!
The doc at the CDC whisperd in Rick's ear that the virus is already inside of them so when they pass on they become a walker. That's why Shane came back.

If you listen ( which you 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 you die you'll become a zombie unless your brain is destroy. So listen the walking dead is a listening and watching show. Duh!

Everyone in the Walking Dead Universe is already infected. It's standard zombie apocalypse knowledge.

The virus is inside everyone. If you die from anything (disease, old age..) That doesn't involve walkers, you turn into one anyway. If you die in the Walking Dead universe, you turn into a walker

The answer is you're an idiot.

the disease is airborne because that's how Randall became a zombie because when Daryl or 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 or whatever she learned how to fight the disease on her own.
if she gets bit or 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