The Guard Post

Sergeant Major Seong-gyu Noh, along with a friend in the army, are paying their last respects to Noh's wife. Later, in their car, Nohâs friend informs him of an assignment that the military has ordered him to do: head to Guard Post 506 to investigate a strange incident there.
Noh arrives at GP 506 about an hour later. On the way to the office, he passes a boiler room that has words written in blood on the wall, which says: âKill all of them.â He is brought to the recreation room where the bodies were found, and watches in shock at the amount of blood splattered around the room.
In the present, Noh checks the guard postâs records and notices that someone has been controlling the weapons lately. All personal weapons were kept in the armory and not in the barracks, which was unusual. One of the soldiers reports to Noh, informing him that they have found another survivor, inside the generator room.
Noh reports to his superior on the phone, who wants him to bring Yoo back, despite his objections as the investigation has not been completed yet and will be forced to end once the clean up crew arrives in the morning. Noh refuses to listen anymore to his superior and ends the call. He then goes off to look for Yoo at the canteen, only to find heâs not there.
Noh reads Kangâs personal records that he was a troublemaker, but he thinks that Kang doesnât look like one who would go insane and embark on a killing spree. He pores over Yooâs journal.
In the present, Doc returns with the bodies and Cpl. Kang lapses into critical condition. Despite Docâs attempts to resuscitate him, Kang dies. As they talk, Kim supposedly saw the dark silhouette of what he thinks is Sergeant Ma walking towards the medic room where they were. He went hysterical and pointed the shadow out to Yoo.
Doc is unable to find any cold medicine, which supposedly should have sufficient amounts of it. Meanwhile, Cpl. Kwak and his buddy are sent for sentry duty.
In a matter of hours, the disease that inflicted the men of 506 will be present in their men as well. Doc surmises that a rabic-like virus infects the men, making them turn violent and kill each other, but he does not know how the infection spreads.
Noh, Doc, 1st Lt. Bang and the sergeants gather the remaining men for an inspection due to the disease. Doc divides up the men according to whether they have the rashes on their bodies. However, when Noh orders the infected to strip their weapons and hand them over, they refuse and the soldiers present have a standoff. Meanwhile, Kwon, tied up in the generator room, struggles with his bonds and knocks out the electric supply temporarily, blacking out the bunker.
Noh, upon learning the truth from Kwon, beats him up in anger, due to him withholding the truth and his own selfish reasons of wanting to live, have caused his own men and Noh's group to die. Kwon retorts back, saying that HQ did not care about them when they were dying like mad dogs here. Later, while staring at the rain outside, Noh touches his family photo and comes to a terrible resolution. He goes into the office and finds the keys to the bunker. When leaving, he hears sobbing sounds from a corner, to find Sergeant Yoon. He discovers Yoon is infected.
Lee orders the men to look for Noh, who is busy spilling kerosene all around the rooms, passageways and corridors of the bunker. Lee and the soldiers manage to corner Noh in the shower room. Unable to kill Noh, Lee wants him to join them and deny that the disease ever happened.
Noh and Doc manages to kill 3 of the men, with Noh injured. Doc runs out of ammo in the end and is shot to death by Lee. The soldiers approaching the bunker instantly drop to the ground for safety, while GP 506 has turned into a smoking ruin.

Question: What has infected the men?
Answer: virus