Zombie bites are not how the "infection" is spread. Instead, zombie bites and saliva cause infections that are 100% lethal. Characters have speculated, but cannot confirm, that whatever causes zombies to re-animate is some sort of virus that everyone in the world is technically already infected with but is only triggered by death. It is known, however, that upon death, it can take anytime between three minutes and eight hours for the corpse to reanimate. The rule is: whathever it is that causes the zombies, is something everyone already has. If you stub your toe, get an infection and die; you turn into a zombie. Unless your brain is damaged. If someone shoots you in the head and you die... you're dead. A zombie bite kills you because of infection, or blood loss... not because of the zombie "virus".