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It seems like whenever the headcrab attaches itself to a host, it keeps the host alive and pretty much feels all the pain the headcrab might, but more since its mostly a human body. So therefor, whenever the zombie catches on fire the host screams BLOODY MURDER inside of the headcrab. Isn't that really creepy? I can only guess there is some oxygen in there. :|
Oh, whenever the headcrab detaches itself it takes his head along with it. That's what the teeth are for.