Smugglers move people, guns and contraband past FEDRA in the Boston quarantine zone. Joel and Tess are smugglers themselves, and the rival crew of Robert, an arms dealer, is the first group of humans you fight in The Quarantine Zone.
They are the weakest humans in the game and work as a tutorial for melee, cover and stealth. Robert's men are behind on pay and unhappy, which you can overhear while you clear the warehouse.
Joel and Tess corner Robert at the wharf. He admits he sold their guns to the Fireflies, and Tess shoots him.
FEDRA
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.
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.
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.