HEV Scientist Zombies lol
headcrabs would need living hosts to become zombies, and the only headcrabs you see are the baby ones and gonarch, which can’t do much.
plus, all the potential hosts are dead.
No clue where you got the living host thing, ravenholem in hl2 proves that wrong. the place was killed off long before freeman arrived.
Those could have been genetically altered by the Combine though. At least I’m pretty sure the venomous/fast headcrabs were genetically engineered. But ignoring that - what are the headcrabs doing in Xen anyway? What’s their role in the ecosystem? What other creatures (besides humans) can they latch on to? We’ve seen them get eating by barnacles and killed by bullsquids. I doubt they stand a chance against houndeyes either (where would they even attack). So - are they at the bottom of the food chain in Xen? What do they even eat when they don’t have anyone to latch on to? Xen plantlife?! They gotta be surviving SOMEHOW… right?
All my crabs head crabs lice are Non-GMO.
I don’t think that’s entirely true. Throughout Ravenholm there are tons of headcrab shells. I’d assume the Combine launched those before any other experiment or attack, which would be where the abundance of zombies and headcrabs came from.
there is also the fact that the first headcrab you see on someones head is on a dead guy.
But then you have to consider the multiple scenes throughout Half-Life and HL2 where a headcrab latches onto someone’s head, and they drop on the ground and stop moving. My theory is that it takes time for the headcrab’s control to take full effect, and the body is paralyzed in the meantime.
lets consider what we know, the zombies in half life 2 are genetically modified by the combine so for them I am pretty sure they would leave the victim alive as a scare tactic for the combine. In half life one though they are wild animals and a parasite that controls the host in nature at least tends to kill the host at time point.
For headcrabs I would expect that they can take both a living or dead host so long as some requirements are meant. AT the least on the dead I would say that they have only been dead for a few hours. while living at any time.
This is one of the instences where I curse valves not making any real lore for their games. AS neither of us can know if we a right or wrong and knowing valve they would say something stupid like yes both are right.
Anyway it doesn’t change the fact that a zombie in a HEV suit would be a cool enemy to face zombies are rare in the later part of the game and a threat bump up one that is also rare would be nice.
I can’t imagine how headcrabs wouldn’t kill their hosts shortly after latching, either from massive trauma or suffocation (as seems to happen in the conference room scene). I have no problem ignoring the HL2 zombies as they’re not very consistent with the HL1/BM version and don’t make a great deal of sense (how come all zombies have white shirts, even though the standard citizen uniform is blue and the resistance fighters wear grey? Will wearing a colored shirt cause headcrabs to avoid you? Is that why everyone wears blue???), but in HL1 we see headcrabs on dead bodies so I think it’s pretty clear they can indeed do that. Another thing to note is that, if headcrabs could only possess live hosts, then as the disaster went on and the number of living scientists and security personnel decreased, we would see fewer zombies overall and more of them would be zombie Marines. That does not occur (most prominently in Surface Tension).
I think that all HEV teams sent to XEN wouldl have helmets equipped, it would be very rare if headcrab managed to get into suit. But if do, lets hope they don’t know how to use HEV suit chargers. 
I can already imagine deadly combo charged HEV suit + longjump zombie jumping toward you from dark cave 
well headcrabs can get through military helmets and then there is what ever material the combine used.
but yeah, that would be the big threat a zombie that takes reduced damage from all sources. don’t think they would be able to actively use the long jump. though I had an idea somewhere for ZEVs/HEZs.
EDIT FOUND IT
:Z.E.V:
info: zombie HEV, these would be unique to xen and would only appear in
areas that show signs of scientist camps, they would come in two types
standard and long jump.
looks: do to the HEV constricting the head crabs mutation, the suit is
bulged in the chest and arm areas, with large bone like protrusions
breaking out of the forearm, shoulder, and back areas. With out the
headcrab one can see that the head and helmet are ripped to peaces, with
the helmet being torn off and the skull being almost devoid of fesh,
infact it is missing the lower jaw and has no brain to speak of with a
large hole in the head.
:function:
the Z.E.V has two functions, first being an endgame version of the
normal zombie, slow and weak but a threat in numbers, the biggest
difference is the fact that they have a random “shield” level that
reduces damage by 1-15% and will drop ammo canisters which can be seen
on their back if they have one.
The second function would be the long jump ones, these would like the
zombine in a sense as at random do to the punctures in the pack the long
jump may activate speeding up the zombie and allowing it to jump after
the player. though the zombie has no control over this, and it is just
as likely to activate mid swing as it is upon spotting you. (just
imagine a Z.E.V taking a swing only for its long jump to activate
forcing it over a cliff.)
I’m pretty sure the Zombies in HL2 contain living humans, as we can still hear the noises of the hosts the headcrabs latch on to (i.e. vanilla zombie fire pain sounds or zombine radio transmissions).
Plus, the headcrabs seem parasitic in that they take control of the host and look for other hosts when it dies (esp. in HL2, where if you shoot the host instead of the headcrab, the headcrab can still attack after the host is long dead).
Actually its just speculation as the none combine ones tend to loop things so that is likely the headcrab using the vocal cords which in its self is crazy. (the host can’t be alive at the point do to some of the mutations being things that would outright kill a person).
On the zombine though, yeah they are likely to be alive purely on the fact that the combine might have removed the human brain and replaced it with a computer in the chest. but once again we do not know for sure. And on the looks for another host when the original dies, reminder that many parisites in games will jump out of the host when the body is to heavily damaged to maintain function. As far as we know the headcrab might be jumping off because the body is damaged beyond use not because it is dead.
Remember their are only 2 scenes that show headcrabs taking a living host one in half life with the guy who kills one with filing cabinet, and one in half life 2. Yet we do see headcrabs using dead hosts in half life, OP4, and half life 2.
I really think the only thing the headcrab needs from its host is the ability to connect to the nervous system. which is why the only bodies you see in areas with headcrabs are burned, or implied to have destroyed their brains. (like the dead bodies infront of a tv). Hell even the wiki doesn’t help with this stuff as there is info supporting both sides.
If you play the Zombie sounds in reverse when they’re on fire, they scream “KILL ME!”, so I’d say they’re alive… Or the Headcrab knows english and wants to stop the burning.
youtube.com/watch?v=LXuR41eisnM
If the host bodies were already dead, then why do they die again when we shoot their bodies and leave out a loud dying scream?
exposed lungs, and a pressure gap can cause that.
As for the zombies screaming, do to it being backwards talking we don’t know if that is the head crab using the body to scream or the host being alive. the only reason I don’t think the host is a live is they only scream when burning and that is the only damage that would be hurting both the headcrab and the host. its obvious by the animation they can play when burning that they understand fire is bad but if the host was alive I doubt they would be asking for help and would likely be asking for death.
Sorry for the double post…
you can spawn npc_houndeye_suicide and npc_houndeye_knockback as well as the baby headcrab in console now. though all three look like this.
All three are immobile and the headcrab makes no sounds. the knockback hound eye does nothing do to not having anything. while the suicide sends a shock wave on death and dies in 2 shots from the mp5. oh and the knockback one seems to have 3x the health of a normal houndeye.
Why would they be screaming anything at all in reverse?
Good points, tho the half-life wiki mentions that Zombies in the game doesn’t care about fire and will just walk straight through it, so the head either the headcrab only understands the concept of fire damage = bad, or it doesn’t know of the concept of fire at all and the host is the one doing the screaming.
When you have a headcrab down your throat, your brain doesn’t work in a good functional way. 
been messing with the houndeye suicides. (you can spawn them with npc_create npc_houndeye_suicide).
While they do spawn as an error, have no AI and do not move, they do have their death explosion and it seems to do less then a normal one. that said it is fun to kill headcrabs with them.