To me it seems that the mechanism by which zombies infect others is self defeating.
Starting with the classic "Romero" zombies, the zombies want to eat brains. However, in order to kill a zombie, you need to destroy the brain. So, every human that undergoes a "complete" zombie attack will be incapable of being a zombie, because the brain will have been destroyed by being eaten.
In The Walking Dead, the zombies seem to attack with the intention of consuming as much of the victim as possible. Assuming they prefer live flesh to dead, then at first there would be more humans than zombies, meaning attention would constantly turn to the next available living person (though given a human could stay alive during an attack, that could be a while).
However, after a point, the number of victims gets reduced to the point where the zombies would swarm and consume individuals almost entirely (as was done to the horse in the first episode). Lots of dead people, but the number of new zombies would taper off.
Also note that in the second season, when Shane wanted to get away from zombies pursuing him, he shot Otis so that the zombies would turn their focus to the guy who couldn't run, indicating that they will focus on consumption more than pursuit, which reduces the chances of spread even more.
In a scenario of "live" zombies, like 28 Days Later, the infected only seem to attack until the person being attacked shows symptoms. This might be the most plausible form of infection, but unlike undead zombies, it seems to me the people would sustain broken limbs, gouged eyes, and fatal wounds, hindering their ability to really become the threat that they are depicted as being.
It seems that the zombie infection spreads only through incomplete attacks, which doesn't seem likely to spread rapidly. The more a zombie is successful, the less likely they are going to create a new zombie.
Is there some generally accepted rationale that can explain this away? Or am I mistaken in some part of my understanding of how the vectors of a zombie infection would propagate?