According to how the model is named, the black headcrabs are called "poison headcrabs" just as the zombies carrying them are called "poison zombies". I for one hated how they poison you for like 70dmg which is healed by your suit slowly. That way you have to run like a madman till it heals. Good thing they cant kill you, they can only bring you to 1hp, but if there's something else around when they hit you...ugh... <_<