The platoon is working in slightly more stressful situations than usual; because of rumors about increased North Vietnamese troops, they move only at night. It's called the night life.
Everyone is tense, but for Rat it's the worst—maybe because of all the bodies he's seen as medic.
First he's just quiet, but then he starts talking about the bugs in Vietnam and how crazy they are, and how they're all after him. He can't shut up about the bugs.
He starts scratching his bug bites compulsively.
The night life makes everyone feel a little crazy—in the dark, the countryside feels like it's alive—but Rat isn't even sleeping during the days, and he just completely loses it.
He tells Mitchell Sanders that he can't shut off the part of him that's a medic, that when he's talking to people, he starts to picture them dead, or in pieces. He sees his own body, covered in bugs. He starts to ramble about the men who have died.
So he shoots himself in the foot. Cross lies for him, saying it was an accident so he doesn't get in trouble. Rat is airlifted to Japan, and he's out of the war.