The basic Doom 3 Zombie is a reinterpretation of the classic Former Human. They were once UAC civilians, now possessed by evil spirits. A scripted event early in the game shows scientist Jonathan Ishii coming into contact with a transparent "ghost skull", which turns him into a Zombie. The demonic force that creates Zombies does not differentiate between the living and the dead - slain humans will be re-animated as Zombies, and do not seem to be any less powerful than the ones created through possession of a living host.
Zombies come in differently-colored clothes, unlike the original Former Human soldiers, but have other minor characteristics in common with the classic enemies, such as one zombie type having withered hair (compared to the classic Former Human's green hair). The basic Zombies in Doom 3 do not carry firearms like the Z-Secs, though a few may use wrenches and flashlights as clubs.
They generally come in a number of flavors, each with different heads (or even, occasionally, no head) and different damage skins.
They stop appearing after the marine returns from Hell.
The basic zombie types take around eight pistol bullets to the body to kill (in most cases, 6 shots should kill), but headshots do twice as much damage. The flashlight melee attack can finish them in 2-3 hits. If you are short on ammo, use the Chainsaw. They move rather slowly and are melee enemies only. However, their melee attacks do much more damage than Imp attacks, especially if they are armed with a flashlight or wrench to use as a club, they can end up hitting the player hard if they end up pinning the player against a wall.
They come in somewhat different varieties, with the Fat Zombie a bit harder to kill due to its higher hitpoints, and the Headless Zombie due to not having a head that can be used to put it down more easily. Weapon-carrying zombies are even more difficult, in particular Chainsaw Zombies, Z-Secs (pistol, shotgun, or machine gun), and Commandos.
Mostly slow-moving and entirely melee-only, these monsters are really only in the game as target practice and for putting a scare on the player when they burst from a "monster closet" early in the game. Any weapon, including fists and the flashlight, can be used effectively against them, but when a melee weapon is used, it is necessary to take careful aim and back away frequently to avoid their attacks. On the other hand, when facing a Chainsaw Zombie, forget melee - it is vital that you kill the zombie before it can attack you at close range.
When facing multiple zombies, the Shotgun is recommended on these walking corpses. The player could maneuver tactfully and shoot down zombies with ease. However in the case of limited shells, the player should switch to the pistol or machine gun instead.
Zombies are ideal enemies to use the pistol or the chainsaw on. When facing a Chainsaw Zombie, you should use a rapid fire ranged weapon instead of weak or slow firing weapons.
If you notice a dead body on the ground, club it with a flashlight. Some zombies play dead, and this tactic will save the hassle of being ambushed by one.
The Chainsaw Zombie can also be killed by standing still with the chainsaw spinning, and letting the Chainsaw Zombie walk into the spinning blade and kill itself. If you instead run at a Chainsaw Zombie with the chainsaw, the zombie will be able to hit you with its chainsaw before it dies.