The BackGround -
Once a Headcrab successfully attaches to a host, the Headcrab will assume control of all their motor functions, able to walk and attack with the host's arms and legs. Through some unknown biological process, the host's hands turn into long, skeletal claws and a massive hole appears in their chest lined with the hosts' ribs, becoming a makeshift "mouth", with their ribs serving as teeth (zombies have been observed to feed by ripping chunks of flesh off of corpses and stuffing them into this maw) Removal of the Headcrab reveals that the host's head is bent abnormally upwards, their hair is matted with blood, and their face is pale, with their eyes shut and mouth open in a frozen scream. In Half-Life 2, the muffled pleas and screams coming from the zombies indicates that the victim may be somewhat aware of their zombified state and obviously in an unimaginable amount of pain the entire time.