"What the hell are these things? And why are they wearing science team uniforms?"
― Black Mesa security guard

The Standard Zombie is the most common enemy seen in the Half-Life series.
It goes by many other names such as "Mawman" In the 1998 version of the Half-Life Prima Guide, "Crabhead" In the book The Zombie Recognition Guide, or referred to simply as a "Zombie". It's a form of a Standard Headcrab latching onto a host's cranium, taking over their nervous system.

Faction:
Xen wildlife

Type:
Parasitized human

Health:
50/50/100
25/40/70 (Half-Life Dreamcast)
50/50/100 (Zombie security guard)
60/60/120 (Zombie soldier)
50 (Half-Life 2)
18/45/45/60 (Half-Life: Alyx)

Attacks:
Claw swipe (10) (5/20/20/35 Half-Life: Alyx)
Double slash (25) (8/30/30/45 Half-Life: Alyx)

Designed by:
Ted Backman
Derek Jenson (HL2 animations)

Voiced by:
Marc Laidlaw (HL2)
Tom Taylorson (HL:A)

Entity name:
monster_zombie (HL1)
monster_zombie_soldier (OpFor)
monster_zombie_barney (OpFor)
npc_zombie (HL2 standard, HL:A)
npc_zombie_torso (HL2 torso)

Once a Headcrab successfully attaches to a host, the Headcrab will assume control of all their motor functions and become able to walk and attack with the host's arms and legs. Through some unknown biological process, the host's hands turn into long, skeletal claws, and a massive hole appears in its chest lined with the hosts' ribs, becoming a makeshift "mouth", with their ribs serving as teeth. Zombies have been observed to feed by ripping chunks of flesh off of corpses and stuffing them into this maw. Removal of the Headcrab reveals that the host's head is bent abnormally upwards, their hair is matted with blood, and their face is pale, with their eyes shut and mouth open in a frozen scream. In Half-Life 2, the muffled pleas and screams coming from the Zombies indicates that the victim may be somewhat aware of their zombified state. This is made more prominent in Half-Life: Alyx, where zombies can be heard begging for help.

Occasionally, Classic Zombies may survive after losing both legs and their lower torso, and continue to crawl using their arms. In most cases, a Headcrab remains attached to its host until the Zombie is killed. Although, in Half-Life 2, they will occasionally detach from the host if it is rendered immobile by its injuries.

Half-Life: Alyx introduces a new characteristic to Zombies in that some have Xen growths protruding from their limbs. A form of the Bloater, these glowing green growths are highly volatile and act as weak points as they explode upon being shot, instantly killing the Zombie. This is an effective technique against Zombies controlled by Armored Headcrabs as the Headcrab's bullet-proof shell prevents head shots. The zombies in this game have a great number of clothing variations, ranging from normal citizen outfits, Combine Hazmat Worker suits, zoo workers, security guards, and even Combine Ordinals.

Trivia:
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- In Half-Life, zombies are seen "feeding" from dead scientists and soldiers by taking flesh from corpses and stuffing it into their chest "mouth."

- In the non-HD variant of Half-Life and its expansions, a zombie's Headcrab appears to be "fastened" tighter to the victim's face, revealing the eerie outline of a human skull.

- Several of the Classic Zombie's sound effects in Half-Life 2 appear to be stock sound effects of leopards.

- In Half-Life 2, the zombie model possesses an animation of the headcrab tearing itself from the head of the host. This is linked to an unused behavior where Headcrabs release themselves from the host when the zombie's health drops below a certain level. This behavior's remnants can be seen and re-added through alterations to the zombie source code in the Source SDK.