The headcrab is a small, crab-like creature that is parastic in nature. When exposed to a human, a headcrab will attempt to latch onto the skull of its victim, gaining assumed control of all neurological impulses, mostly those to do with movement. Those infected by a headcrab are referred to as zombies.
Headcrabs have been harnessed by the Combine in the form of 'Headcrab Shells' fired from sites placed within the Outlands. The city of Ravenholm was bombarded by these shells, which resulted in its destruction and abandonment.
Headcrabs are known to have come to Earth from Xen, but, as Xen contains a mixture of many other-worldly creatures, the true origin of the headcrab remains unknown. Vortigaunts are known to eat them. There are three known subspecies of headcrabs, listed below.
Standard Headcrab
The standard headcrab is the most basic of all three subspecies. It moves at a brisk pace, and attatches to the heads of victims as expected, with no special adverse effects. Headcrabs will often attack their prey with little consideration of threat, which has brought the assumption that they lack self-preservation skills like the poison headcrab.
Fast Headcrab
The fast headcrab is similar to the standard headcrab in that it latches onto a single humanoid target's skull and gains control. However, due to its notable leanness and lack of body mass, a fast headcrab, as its name implies, is much faster, demonstrating much better movement speed and agility. When attatched to a host, a fast headcrab will alter the body of its host in drastic ways in that the majority of body mass and fat will be expunged, leaving exposed muscle tissue for a very lean, agile form. This allows fast headcrab zombies to move, climb, and manuever at unmatched speeds.
Poison Headcrab
The poison, or black headcrab appears to be the most intelligent of all three headcrab types. This creature will, like all other headcrabs, attach itself to human hosts, however, unlike the others, many poison headcrabs can be attached to the same hosts, forming 'colonies' on the backs of said hosts. The victims of this creature will hunch over to allow up to five poison headcrabs to cluster on its back. Poison headcrabs, as their name suggests, contain a deadly neurotoxin within their fangs. The true nature of this poison has yet to be revealed, although it is assumed that it could quickly, if not instantly, kill a person without an andidote. Unlike the other two headcrab types, poison headcrabs are able to stalk, retreat, and reason when hunting. Because of this, they are thought to be the most intelligent of the headcrabs. They also have a sparse covering of coarse, black 'hairs.'