I believe this is what’s going on in The Walking Dead, Sounds about right.
Zombies, also called Roamers and/or Lurkers/biters in the comic series, Walkers and/or Geeks by T-Dog and Glenn in the The Walking Dead (TV Series) are the main non-human antagonists of The Walking Dead.
The zombies are the classic slow walking Romero style zombies, which originated in Night of the Living Dead. Following classic Romero-rules, everyone who dies, for any reason, will re-animate as a zombie except if the brain is damaged/destroyed. Even someone killed by a gunshot wound with no zombies involved at all will re-animate as a zombie. 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 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.
Zombies out-number humans around 5,000 to 1.
…the rule is: WHATEVER 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.”
The most common way that people become a zombie is after dying from infection caused by a zombie bite. Symptoms of a zombie bite include: