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The HeadCrab is an alien life form form a race called Xen. Xen is a race of aliens which were transported through a portal which was caused by Gordon Freeman in Black Mesa. The Head Crab walks on 4 legs and leaps great distances.
Now there are many ways to describe the HeadCrabs zombification on the host, but no one knows for sure. The basics are that the head crab attaches itself to the hosts head. Then the headcrab uses its "hidden" beak no the hosts skull which paralyses its host which makes it easier for the zombification process. After 60 seconds the headcrab uses its nerval system and attaches it to the hosts nerves, and after the brain has been fully compromised, the zombie will get up. Also the Host isn't dead for sure when the zombie dies. If the headcrab is shot off, it takes the nerves out with it which practically kills the host.