Each enemy type within Half-Life 2 fulfils a role, its behaviour designed to provide a different form of threat. Standard Headcrabs present a minor threat, with weak attacks and low health they are relatively easily dispatched. Fast Headcrabs are slightly more threatening due to their increased movement speed and the difficulty that creates for targeting them. Overall, Headcrabs serve to provide light harassment during periods of exploration or puzzle solving, with one exception.
Larger and slower than their brethren, the squat black Poison Headcrabs nonetheless pose a unique threat that alters the dynamic of any situation in which they are encountered.
Like others of their species, Poison Headcrabs leap to attack and though their standard bite causes twice the damage of other Headcrabs, it is the secondary effect of this attack that makes them stand out among the fauna of City 17. The bite of the Poison Headcrab administers a powerful neurotoxin that immediately causes your health to drop to one. Though the anti-toxins supplied by your HEV suit will counteract these effects, restoring you to health (minus that caused by the initial attack) this process is not instant; during this period even otherwise minor damage from additional attacks or environmental hazards can be enough to kill you.
Along with their increased size and slower movement speed, Poison Headcrabs are differentiated further by their distinct black colouration, and the unsettling rattlesnake like noise they emit when searching for prey; nothing else in Half-Life 2 sounds like a Poison Headcrab. This differentiation is so strong that upon hearing their distinctive audio cue players in playtest sessions would abandon their current priorities to immediately search for and kill the Poison Headcrabs.
This aspect of player behaviour is exploited within certain parts of Half-Life 2 to create spikes of tension in situations where there is little or no direct threat. Despite their inability to kill you themselves a single Poison Headcrab can increase tension even when not supported by other enemy types.
The threat presented by Poison Headcrabs is augmented by the existence of the shambling Poison Zombies, a bloated shambling figure with multiple Poison Headcrabs clutching its back. These passengers will leap to attack when they perceive a threat and though they cannot kill you themselves they will distract and disorientate you, potentially allowing the Zombie itself to close the distance and use its slashing melee attack, which can easily kill you if you are recovering from the venomous bite of a Poison Headcrab. Poison Zombies can take three times as much damage as other types of Zombie making encounters with them some of the most tense in the game.
Distinctly identifiable by sound, shape, animation and colour the presence of even a single Poison Headcrab alters the dynamic of play. More than simply a means of harassing the player, Poison Headcrabs serve as amplifiers, creating a spike in tension and increasing the danger posed by other enemies. Evoking a sense of fear and panic out of proportion to their ability to inflict damage, the knowledge that one or more are nearby causes a shift in priorities, making even an otherwise low risk encounter tense and threatening.