Zombies

Zombies are the central antagonists of the films Seoul Station, Train to Busan , and Peninsula. They are humans that infected with an unnamed bio-engineered virus which become extremely violent and act as a vector to spread the virus by biting others. There is no evidence that they actually eat the people they bite. The closer the bite is to the head, the faster the transformation.

Origin
The unnamed and unknown virus is a deadly artificial mutagen developed presumably unintentionally by Seok-Woo and his co-worker. This mutagen created by the Biotech company has resulted the contamination of natural water and first infected the animals like the deer in the opening scene which reanimated after being ran over by a truck. The virus then shortly and quickly spreads to the cities of Seoul and then throughout the rest of South Korea. The Korean military established evacuation centers in Daejeon and Busan. However, Daejeon never stood a chance and became overran by the infection as military personnel and police are all infected then Busan is the last city standing among the rest and becomes a successful evacuation center for refugees and survivors until it is no longer safe due to a plane crash.

Characteristics
Like most infected or zombie movies, the virus spread through bites or scratches and then it takes the victim a few seconds or possibly minutes to become infected. Possible injection of infected blood or bodily fluid may contract with the virus. The symptoms of victims prior to infection are convulsive seizure and twitching. After infection, the infected may hunt down another uninfected victim with their ferocious speed, agility, and with a mob mentality. When the infected are bunched up as a horde, they have a tendency to trample or run over each other just to chase down an uninfected victim. They're very persistent when it comes to attacking as they attempt to reach them through a glass window or by falling through a ledge. Their main weakness is darkness, low-light areas or being blinded as they cannot see their victim very well and only reliant on sounds, making maneuvers easy. Even in lighted areas, they are also attracted to sounds made either from survivors or from another horde. Another one of their weakness is dismemberment or broken limbs, which in turn can slow them down. As demonstrated in the ending scene by military personnel, extermination of these zombies is only the destruction of the head or brain. In rare occasions, the lifeless infected can still twitch despite the headshot wound, implying that the infection is still in the body. Sometimes when a horde is being bunched up and being ran over, they unintentionally "kill" each other due to suffocation and physical trauma.

Trivia

 * The zombies in Train to Busan and Peninsula bear feats similar to fast zombies/infected from films like 28 Days Later, REC and it's inspiration World War Z.
 * There's no possibility of a cure or an infected victim becoming an asymptomatic carrier.