Zombieman

The Trooper (also known as the Zombieman) is a marine who has been turned into a zombie during the invasion of the UAC bases. He appears as a green-haired man in bloodied utility uniform, carrying an assault rifle.

The Doom instruction manual says:
 * Just a few days ago, you were probably swapping war stories with one of these guys. Now it's time to swap some lead upside their head.

In the instruction manual the trooper is called a Former Human.

The trooper is first encountered on E1M1: Hangar (irrespective of skill level).

Combat characteristics
The trooper attacks by firing a single bullet, which is slightly less powerful than the player's bullet. He is generally considered the least dangerous enemy in the game; no other monster (excepting Commander Keen) has a lower movement rate, hit point total, or average damage per shot. When killed, he drops an ammo clip containing 5 bullets (10 on I'm too young to die and Nightmare!).

Tactical analysis
A healthy player should never back down from a single trooper, although large groups can slowly drain the player's health during the time it takes to kill them all (as with MAP09: Stronghold or PET2.WAD from Maximum Doom). His aim is somewhat unreliable even at medium range; consequently, he often finds himself outgunned at long range, even by a player with a pistol. In an emergency, or when conserving bullets during a Tyson run, the fist is somewhat effective as long as the player's footwork is good (two to three blows, or one berserk attack, will knock a trooper down).

The trooper's low rate of fire and damage capability make him a rather weak contributor to monster-monster battles. Attrition among a crowd of troopers is generally slow, and distraction of larger monsters minimal.

In many stock levels, most of the enemies in the first few rooms are zombies. Unless the player's primary goal is speed, their dropped clips can be a valuable early source of ammunition.

Doom RPG
In Doom RPG, the trooper appears as the "Zombie" monster class. There are three different forms, identified by color:
 * Zombie Private (brown)
 * Zombie Lieutenant (blue)
 * Zombie Captain (normal colors)

Inspiration and development
The Zombieman in Doom 0.4 looked almost identical to the Doomguy, the only difference being his red eyes (to show that he was possessed). This sprite had only one frame, and no rotations. In Doom 0.5, however, the trooper sprites were virtually identical to the finished versions.

Appearance statistics
The IWADs contain the following numbers of troopers: