Poison Headcrabs and Poison Zombies are from Half-Life 2. Poison Headcrabs are darker and cat-sized and spider like creatures. If a Poison Headcrab bites you, your health will drop all the way 1 percent! However, Poison Headcrabs alone can not kill you, other enemies around finish you off. Hide if a Poison Headcrab bites you, so your health returns to it's normal levels. Poison Zombies are hunch back Zomnies with Poison Headcrabs around it. Poison Zombies are very slow, but strong. They can throw Poison Headcrabs at you (only 2-3), Poison Zombies have large health, use your shotgun alot of times for fire your Assault Rifle at them. Really use a exposive barrel or gas can to beat a Poison Zombie.
Poison Headcrabs and Poison Zombies are more poisonous then normal ones. Poison Headcrabs can be used the same way as normal headcrabs, Poison Zombies are strong and do not come in Torsos.