The Poison Headcrab's assimilation of its human host results in the host body becoming bulky and bloated, taking on a reddish hue as it bleeds from its pores. The flesh on the host's back recedes and tears, exposing the spinal column. With its additional weight, the Poison Zombie is incapable of standing upright and will walk with a slouched posture. It is slower than other zombie types, but also considerably tougher, having three times the strength and endurance of standard zombies.
Poison Zombies are not actually poisonous themselves; they will lurch towards prey and attempt to slash at them with their mutant claws. However, they often carry other Poison Headcrabs around with them which latch onto the zombie's body. When prey is near, the zombie will grab one of the headcrabs clinging to its body and throw it at the victim. The thrown headcrab will attempt biting its prey, secreting a debilitating neurotoxin into the victims' veins.