"Infected" is a term that refers to the reanimated corpses of recently deceased people. They serve as the universal antagonists of FtWD.
Exactly how the bodies of the recently deceased reanimate is not understood, and is not explored within the show. It is implied that the bodies are infected with some kind of agent, (with a heavy emphasis on the possibility of a virus), that reactivates certain primitive areas of the brain after death, and through this, the basic motor functions of the body are restarted as well as the most basic need -- the need to feed, which manifests as their sole drive.
The effect on society of the appearance of the infected is complete chaos. No government agency knows what the infected are or how to deal with them. Planes are grounded and police waste ammunition by shooting the infected in the body as per their training, such as in the case of Patrick Sutherland. Also the apparently reckless shootings draw the ire of the general public, as they are also unaware of what is happening and, taking the shooting of the infected as police brutality, begin to riot. This is seen in "So Close, Yet So Far" and allowed the chaos to spread, accelerating The Fall.
Robert Kirkman has stated that the origin of the undead apocalypse won't be revealed.. "any explanation would disrupt the normalness, and [change the feel of the book to resemble] 'science fiction'." He is more interested in what happens post-outbreak than what had happened leading up to it.
Everyone appears to be "infected" with the agent that causes reanimation after death. It's not been revealed whether this agent is a simple chemical, virus, bacterium or fungal spores.
Because everyone is infected, every person who dies - from any cause - will reanimate, unless their brain is severely damaged.
In the trailer for Season 1, the news advert mentions a "mysterious virus" going around, giving a possible hint to the origin of the outbreak.
They also have been known to use intelligence in both FTWD and TWD universes.
After death an infected person's body reanimates and can only be destroyed by severely damaging the brain. Decapitation (cutting off the head) without destroying the brain leaves a head rolling its eyes and snapping it's jaws at whatever comes into range. Thus, the only way to permanently put down - kill - an infected person is to destroy the brain.
Notable Known (and Presumed) Infected
- Gloria The first confirmed infected seen in the Walking Dead universe, and Nick's girl friend.
- Calvin The first infected seen by Madison Clark, and Travis Manawa.
- Matt Sale (presumed) Alicia Clark's boyfriend from before The Fall.
- Infected Rioter The infected seen by Christopher Manawa through the window of Salazar's Cuts, as Chris peered through the window the unaffected man was looking back at him.
- Lumbar Yard Survivor
These names are also used on FtWD:
- Skin-bags - Moyers, Season 1, "Not Fade Away"
- The Dead - Daniel Salazar, and others
- Groaners - Fear the Walking Dead: Radio Waves The Last DJ (See Radio)
- The Wasted - "Do Not Disturb", Brandon's group call them The Wasted, because the first one they saw looked drunk.
- Zombie - Alejandro Nunez uses the term when talking to Nick in Pablo & Jessica. The English word was used in the clip on The Talking Dead, shown the week prior to transmission. The show itself used the Mexican word dubbed in afterwards: "zombi". It seems likely that the use of the word was a mistake that slipped through until the clip was shown on The Talking Dead. The word was also used by the waitress in S03E14 around 22:14, where she referred to the food as "antojitos Zombies".
- Biters - Diane uses the term in "El Matadero" when talking to Alicia Clark
- The Passed - John Dorie in What's Your Story
- Walkers - Morgan Jones in What's Your Story
Behind the scenes
Infected is the counterpart of the term walkers which is used in the parent series The Walking Dead.
- Kirkman eventually did tweet that spores from space caused the infection - but later deleted it.
- Frank Dillane in an pre-Season 1 interview, appeared to slip and say "eaters", before correcting himself and saying walkers.
- In early seasons infected look "fresher" in appearance as the undead apocalypse in Fear has just begun. They look similar to the walkers in the first season of the parent series.
- As the seasons progress, Infected are beginning to show signs of decay.