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The Zombie is a common enemy featured in the Half-Life series. It is created when a Headcrab attaches to the head of a suitable host and takes control over its victim's body, rendering the host a "Headcrab zombie" (or "necrotic", as they are referred to by the Combine Overwatch). They are only seen on Earth, usually in remote or deserted areas.
As their name suggests, zombies do not appear to retain much of their former intelligence and, like Headcrabs, will blindly pursue their prey, oblivious to danger (such as the propeller traps seen in Ravenholm). Zombies do not respond to fire and will walk blindly into it, though they feel it on them and notice they are burning once they are on fire. Despite this, they do seem to be fairly intelligent, as a Zombie HECU soldier is witnessed in Half-Life: Opposing Force to throw a dead scientist it had just killed in order to smash a window to get to Shephard. Also, in Half-Life 2: Episode 2, a Fast Zombie is seen digging through boxes in a dumpster. Throwing a grenade inside will make the Zombie throw it back out towards Gordon.
Zombies mainly attack with swipes from their claws, but in Half-Life 2, they gain the ability to forcefully punt objects towards the player. Also in Half-Life 2, certain Zombies can be found "sleeping" and stand up to attack the player if they get too close or are attacked.
Zombification is also one of the ways used by the Combine to subject and intimidate the human race. Headcrab Shells will often be sent to Resistance camps to dismantle them, turning the Rebels stationed there into Zombies with the use of Headcrabs. This is notable for the Rebel stations along the City 17 Canals (in and beyond the Underground Railroad) or Ravenholm.
A Headcrab's attack does little damage (with the exception of Poison Headcrabs), and it also takes some time for Headcrabs to take over their hosts, who are initially still in control; however, Headcrabs both attack very swiftly and (should they succeed) latch onto the head very firmly, so that any attempt by a struggling victim to remove the parasite with their hands is largely futile. Coupled with disorientation and the imminent loss of consciousness from suffocation, this ensures that most victims nonetheless succumb very quickly, explaining the exceptional effectiveness of the Headcrabs' use by the Combine.
It is not known for sure if a victim of a Headcrab attack can still be rescued once a Headcrab has attached itself onto their head. Gordon Freeman's sole attempt to do this at the infested Station 6 apparently failed, killing both the Headcrab and its victim.
 Behind the scenes
Hostile Takeover, a cancelled expansion pack for Half-Life, was to have Zombie Monkeys. They were the result of Headcrabs zombifying the test monkeys kept in the labs of Allied Processing. They were to be very fast and deal massive amounts of melee damage and act like the Fast Zombies found in Half-Life 2. This zombie type would have been employed to create horror moments, like those in the movie Aliens, potentially seen crawling through air ducts or prowling in dark areas such as sub-basements of the facility.
- On Station 6, a Classic Headcrab can be seen latching onto a rebel's head, turning him into a Zombie.
- When a Classic Zombie is eaten by a Barnacle in Half-Life 2, the Barnacle will eat the Headcrab and drop the host body, killing it.
 List of appearances
- Half-Life: Day One (First appearance)
- Half-Life: Uplink (Non-canonical appearance)
- Half-Life: Opposing Force
- Half-Life: Blue Shift
- Half-Life: Decay
- Half-Life 2
- Half-Life 2: Raising the Bar
- Half-Life 2: Episode One
- Half-Life 2: Episode Two