I don't think they are 'mythical' by any degree. During the groups time at the CDC HQ there was plenty of 'biological' proof, I don't recall if it was a virus or bacterial. Nevertheless, the 'affliction' for lack of a better word, restarts the core of the brain, 'nothing that makes us human' as the scientist said, I wouldn't be surprised if they have some semblance of instincts, much like how some pack animals work together, they might have the ability to work together in a sense. Maybe not in the way a living group of humans can, but they are predators, and predators are pretty sneaky.
Well, zombies are mythical, so the writers of The Walking Dead can make the zombies operate any way they want to.
However, I think the rules are pretty consistent.
1) Zombies are walking dead, but can be killed by massive trauma to the brain, blunt force, gunshots, etc. They are also rotting, so new zombies look relatively human, except for the blood and constant need to gnaw on flesh, but older zombies may be in severe disrepair.
2) If excited, zombies can move quicker and will tend to mass if there's food, and that's either horses, dogs or people.
3) Zombies can see, hear and smell as well as normal humans, but since they are tuned to hunting by this point, they can be very attentive. They're also easily confused by flares and attracted to loud noises.
4) They 'remember' some aspect of themselves, which means they can try to open doors and climb ladders.
5) The infection is through bites or scratches, but not contact.
