FEDRA, the Federal Disaster Response Agency, is the military government that runs the Boston quarantine zone. Its soldiers are the best-equipped enemies in the first chapters, and you fight them while smuggling Ellie out of Boston.
The Quarantine Zone shows soldiers breaking up Firefly cells. In The Outskirts a patrol stops Joel, Tess and Ellie outside the wall, Humvees and infantry sweep the downtown, and squads hunt the group through the Capitol building and the subway beneath it.
Early on you lack the guns and ammo for a stand-up fight, so sneak past or stealth kill. Small groups in the Capitol and the subway only carry pistols and can be taken head-on. In Left Behind, recordings from a Denver squad at the Colorado Mountain Plaza tell their own story.
Infected
The Infected are people taken over by a mutant Cordyceps fungus, and they are the main threat between every human fight in The Last of Us Part I. In Part I and Left Behind you meet four stages: runners, stalkers, clickers and bloaters, each harder to kill than the last.
Image: Naughty Dog / PlayStationBloater
Bloaters are stage four infected covered in armor-like fungus. They are slow and blind but kill you instantly if they grab you, and they throw bursting sacks of mycotoxin from range. Fire is the way to beat them.
Clicker
Clickers are blind stage three infected that find you by the clicking sound they make. Their grab kills instantly unless you carry a shiv and have the Shiv Master upgrade, so most players deal with them quietly or with fire.
Fireflies
The Fireflies are the militia fighting FEDRA's rule and searching for a cure, led by Marlene. Their slogan, look for the light, is painted across the ruins, and their deal with Joel and Tess to smuggle Ellie out of Boston starts the story.