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 breathing.

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 is 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". However, in the explosion at the airport, "they" were were seemingly unaffected even when burning, and the fire did not stop "their" movement.

Trivia

 * Although Saeko Busujima 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 of "them" are killed by simple jabs to the chest. Scientifically speaking, this is probably due to Saeko hitting major arteries and jugular veins causing blood loss within the brain making it lose function, but this is just a theory.
 * It should be noted that some of "them" have retain some expresions. A Good example is in Episode 1, one of "them" looked like he was laughing while in Episode 5, another appeared to have been posing. This could mean that some of "them" retain certain aspects of their personality or their emotions.