On 6 October 1971, American soldier Jacob Singer is with the 1st Air Cavalry Division, deployed in a village in the Mekong Delta during the Vietnam War, when his close-knit unit comes under attack. As many of Jacob s comrades are killed or wounded, others exhibit abnormal behavior, including catatonia and a bloody seizure. A terrified Jacob flees into the jungle, only to be bayoneted in the gut.
Jacob awakens in 1975, on a New York City Subway, with the novel The Stranger. Despite having a PhD, Jacob works as a postal clerk and lives in a rundown apartment in Brooklyn with his seductive girlfriend, Jezzie. Jacob misses his wife and their three sons, especially the youngest, Gabe, who died before the war. He is increasingly beset by disturbing experiences, including inexplicably locked subway exits, faceless vibrating people, a car trying to run him down, a nurse with grotesque teeth on the top of her head, and a bat-like monster penetrating Jezzie at a party, where Jacob collapses with a dangerous fever.
The comrade who had gone catatonic, Paul, contacts him to reveal having had similar terrifying experiences, but is immediately killed when his car explodes. Commiserating after the funeral, the platoon-mates confess that all but one have also been living a nightmare. Jacob explains that the army must have done something to them, and they hire a lawyer to investigate. The lawyer backs out after reading military files documenting that the soldiers were never in combat and were discharged for psychological reasons. Jacob s comrades also back out, looking scared, as Jacob demands they tell him who got to them. Jacob himself is later thrown into a car with intimidating men in suits who tell him to stop nosing around in military business, but he is able to jump from the moving car. Taken to the hospital, he finds himself strapped to a stretcher being wheeled over bloody body parts, past demented, disfigured inmates. Trapped with doctors wielding barbaric metal instruments, he is told he is already dead, but denies it.
He is taken out of the hospital by his friend Louis, who quotes the 14th century Christian mystic Meister Eckhart:
Eckhart saw Hell too. He said:  The only thing that burns in Hell is the part of you that won t let go of life, your memories, your attachments. They burn them all away. But they re not punishing you , he said.  They re freeing your soul. So, if you re frightened of dying and ... you re holding on, you ll see devils tearing your life away. But if you ve made your peace, then the devils are really angels, freeing you from the earth. 
Jacob is approached by a distressed Michael Newman, the man who treated his wound in a medevac helicopter in Vietnam and who dragged him away from Paul s burning car. Newman tells a story of having been a chemist with the Army s chemical warfare division in Saigon. He designed  the Ladder , a drug that massively increased aggression, taking people straight to their most primal urges. A dose was secretly given to Jacob s unit, who then turned on each other in a homicidal frenzy.
At various times Jacob experiences the aftermath of his attack, such as being found gutted in the jungle and being evacuated under fire. At one point he awakens in his home before the war with his wife and all three sons, telling his wife he had a dream he was living with another woman, Jezzie. Eventually he re-experiences the traumatic moment, where it is a fellow American soldier who bayonets him.
Finally given knowledge of what happened to him and why, in a calmer place, Jacob returns to his family's apartment. He sorts through his mementos, including his combat dog tags and honorable discharge. He is surprised to see his son Gabe at the foot of the stairwell, who takes him by the hand and leads him up the steps into a bright light. In a wartime triage tent, military doctors declare Jacob dead after  putting up quite a fight . He has an expression of peace on his face, implying that he is finally dead.
Output: What is the fatal injury that Jacob sustains which ultimately leads to his death?