In the last episode (Better Angels) of AMC's The Walking Dead, Shane and Randall became walkers shortly after dying:
According to Wikipedia:
This episode suggests that the zombie outbreak could be spread through unorthodox and unidentified means
It ocurrs to me that everyone is already infected and they turn into walkers after they die unless they die from some kind of cranial/brain blow. If this is true then scratching and biting doesn't turn you at all but you do get infected.
I don't know if this make sense to you, so my question is: Does anybody have a clue about what is going on here?