Zombies 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% fatal. 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. A corpse can reanimate between three minutes and eight hours after death.
Zombies out-number humans around 5,000 to 1.