Runners are stage one infected, people turned within the last couple of days. They still see, move fast and swarm in packs, so the danger is numbers rather than any single runner.
A lone runner is easy to beat with fists, so don't waste bullets on one or two. Strangle them from behind, never from the front, which makes them aggressive and alerts nearby infected. Save your shivs for clickers. If a pack spots you, sprint away, break line of sight and hide behind cover.
Runners can see you and will scream when they do, which brings the blind infected running. Thin them out quietly first so the clickers and bloaters stay calm while you sneak past.
Some runners stand passive and only attack if provoked, if something lands near them or if you get right in front of them. When startled they shriek, and some look distressed even while attacking.
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.
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.