Them

The undead, more commonly known as Them, are the series' primary antagonists. As of yet, no cause of Their infection has been revealed. The infection appears to be fast-acting, which is why it has spread all over the world in such short time-span. The name Them was given by Hisashi Igou for the fact that They can no longer be considered human.

Physical
They retain whatever They looked like before the people were infected. Additional facial appearances helped in identifying Them. This consists of completely blank, rolled up eyes and tight pupils, uncontrollable tongue movement, wide jaw distance and a pale grey coloration. Bodily, They have the same clothing prior to their infection with a missing part of their clothes or skin, a telltale sign of being bitten by other Them. Although They are undead, they still breathe or at least slouch enough to appear to be beathing.

Intellectual
They have lost all form of intelligence, with the exception of basic actions such as walking, biting, moaning, crawling and, rarely, running. Although blood circulation has stopped, They can still make use of their sense of hearing to determine where Their prey would be. They could also form groups, which can be dangerous to living survivors as They can overwhelm the survivors easily. They can determine whether anyone near Them are infected or not, which can possibly be why They don't attack their own kind, either for three reasons that not only are they capable of hearing ,but smell and considering that they moan. They have been proven to be blind and/or they have some recollection of intelligence left to not attack another infected or they only want fresh meat of humans.

Weaknesses
Although They have enhanced strength and cannot be truly killed by whatever means through the torso and limbs, an attack at the head be it projectile, melee, or well placed hand to hand assaults will truly kill them. While They ignore the pain of strenuous activities and have increased strength, They have lost speed and coordination, only able to slowly walk to their prey. If They fall down, they have a very hard time getting up. They are also incapable of going down slopes, but can climb stairs (Some may find that this goes against the "rules" of a zombie, for zombies can only stumble or crawl up stairs in original sources). While destroying Their brain or spinal cord is the only true way to kill them, They appear to be more flammable than humans and die easily when set on fire. This has proven to be very effective in destroying a medium to large group of Them.

Trivia

 * Although it was stated in the beginning of the series that to fully kill one of Them, a person would have to injure their head and not any other body part, but in the final episode, various Them are killed by simple jabs to the chest