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 Appearance
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.

Strengths
"They" have been described as having superior strength. It can be theorized that due to their lack of a sense of pain, their muscles are capable of passing the limits that living tissue can't handle.

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 seemingly unaffected even when burning, and the fire did not stop their movement.

It has been theorized that they are still like regular corpses, and as such are still subject to decomposition. However this theory has yet to be proven as fact.

In addition to all the rest (though it has not yet been proven by other stories, movies or games) they are (in theory) affected by electricity since the brain still works it can make them numb or kill them since it disrupts the electromagnetic signals from the brain to the body normally humans would require a very large amount of electricity for this to happen thanks to their strange condition they have lost many resistances that normal humans have (for example they can be set on fire very easily).

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.
 * "They" Don't appear to attack animals, only humans. Animals in turn don't appear to fear "Them" at all. The best example is when Zeke barked at "Them", "They" made no effort to attack him despite the obvious noise he made. Although as shown in episode 6, the sound of barking will still attract their attention. Yet when the dog bit one of "Them" on the ankle in episode 8 of the anime, they still did not try to bite him, additionally proving their lack of interest for non-human prey.
 * In the Japanese version of the anime, the cast never refers to "Them" as "zombies", unlike the English version, where the cast calls them "zombies".