Hmmm, so Datu was bitten on the lower neck. Tanya says that's where the zombies typically bite their victims. Now, if they are attacking humans for food then they must kill them before they turn, therefore, like natural predators, they bite in places that will likely kill quickly. Clamping the neck can make a prey pass out, biting the neck can make the prey bleed out. If the human prey is bleeding out, then perhaps most of the infecting agent is not potent enough to enter the body against the spewing blood.

I also wonder if Tanya or Saul will test their zombie invisibly by approaching zombies to see if they are attacked.