In Max Brooks's novel World War Z, "quisling" refers to a human that had broken down psychologically due to the presence of zombies (often referred as Z-Shock) and thus begun acting like a zombie. These humans attack other humans mindlessly but are still attacked by actual zombies who can tell the difference. Despite numerous attempts by government-funded doctors, the psychological trauma of a quisling is too far gone for any chance of successful rehabilitation.
The phenomena is reportedly akin to Stockholm Syndrome. The living human subject becomes so fearful of zombies that they try to "appease" them, or "switch sides", in an effort to become the object of their fear. They begin to act like zombies, moving slowly, moaning, attacking and consuming live creatures, even becoming oblivious to the pain from various wounds they might sustain.
Although similar at first, there are noticeable differences between a quisling and a zombie. First, quislings tend to smell worse than zombies, due to the fact that their sweat glands and other "bodily functions" still work, and the high number of bacteria present on their flesh (absent in Solanum-saturated zombies). Quislings also have a tendency to blink when a light is shone in their eyes, whereas zombies don't. Third, any wounds that they have on their bodies will either still be bleeding or heavily staph infected, again because bacteria and other microbes shy away from Solanum-infected flesh. Any animals in the vicinity of a quisling will generally not panic as they would in the presence of a zombie. Finally, when a quisling is in close proximity to a real zombie, the quisling will ignore the zombie, even when being eaten alive, while the zombie will turn, attack and devour the quisling.
Apart from being a further drain on resources, quislings actually helped spread the misinformation and rumors during the Great Panic that led to the deaths of many people. Initially, video taken of zombies attacking quislings was taken as evidence that zombies attack each other just as much as humans, and that the zombies might do half the work of destroying themselves for us. In reality, zombies only try to eat "warm bodies" - preferably humans, occasionally animals, but never zombies. Worse, the initial quislings which were captured and found to be alive led to the false assumption that the zombie plague could be "cured", when in fact they had never been infected in the first place. Finally, it led to false reports that supported the notion that Phalanx was an effective countermeasure against zombie infection, when in reality the accounts were given by those attacked by quislings, at a time when they were virtually unknown and indistinguishable from actual zombies.
Quislings are also like zombies in the fact that they can keep coming after sustaining a gunshot wound. Unlike zombies, quislings do not have a defense against Hydrostatic Shock, and will also die due to infection or blood loss.
Quislings are distinct from Ferals. Ferals are often children, who were abandoned in zombie-controlled areas at a young age but somehow managed to survive, but as little more than animals. Ferals, however, don't try to act like zombies as quislings do, and are obviously alive, running around, and doing what they can to avoid zombies.
The term "Quisling," originating with Norwegian politician Vidkun Quisling who assisted Nazi Germany to conquer his own country, describes traitors and collaborators. It was most commonly used for fascist political parties and military and paramilitary forces in occupied Allied countries which collaborated with Axis occupiers in World War II, as well as for their members and other collaborators
In contemporary usage, "Quisling" is synonymous with "traitor", and particularly applied to politicians who appear to favor the interests of other nations or cultures over their own. In American English, the term is less well known than the equivalent phrase "Benedict Arnold". Also, in Norwegian, when one removes the "q" and the "i" in "quisling" - the result is "(a) usling" which means "someone who is ignoble or iniquitous". This was used more or less humorously during WWII in Norway.