As long as they can move, they can be dangerous. Remember the emaciated zombies that swam to attack Pegs and Michael on the yacht. They were withering away, but were still able to mount an offense against Michael and Pegs.
Teeth based contraction is unlikely. Tommy was infected after having an open wound infected with blood from the jumper. My vote as of now is blood/bodily fluid based. It's hard to know if a zombie scratch actually infects a human being. To the best of my remembrance we haven't heard a story of someone who got scratched and turned. Everyone we've seen turn did so from blood transfer or bite.
As to the actual discussion: I'm going to throw my hat in for hot zombies. I might even vote for overly hot zombies. I'm guessing that because they are able to accomplish super human feats, their metabolism grows out of control (along with other biological functions) to allow them to do that. This could also explain why the seem to eat so often and so much. The higher metabolism, the quicker you burn through nutrients and need to replenish.