Continuity

Overview
The Walking Dead and it's consistency of persons, plot, objects, places and events have been constantly maintained under ambigious directions by creator Robert Kirkman. Being a long-running series starring an ensemble of characters and highlighting their struggles in an apocalyptic zombie epidemic, the universe of TWD is assumed to be rich in history. However, in both a narrative viewpoint and the writer's choice, the bigger details of the characters and the world they are living in are largely undiscussed or unrevealed, with only little facts and hints shown throughout the series.

Time Period, Divergences and Locations
The exact year(s) the events of The Walking Dead take place is unknown. Robert Kirkman has stated several times in the letter hacks columns that he is no intention of having the cast or himself state specifics, and he dismisses any assumptions of using the comic's release year (2003) as any hint to go by. The comic's 70 issue run has had the character's state most recently that the time past from the beginning to the latest issue has been exactly 14 months. However, no establishment has been made regarding the exact point of divergence within the The Walking Dead (TWD) universe and real world history, or if the The Walking Dead universe had been following an alternate history of it's own before the epidemic. Direct references to such things as particular areas, names, technology or companies and brands are kept to a bare minimum. However, several instances through-out the series in dialogue and passing the reveal parts of the character's surroundings and past. This gives some insight into at least a general guess as to the era of the epidemic.

-When Tyreese and Michonne reminisce on the former's pro-football days in the NFL (which is stated in conversation with Rick as quite some time ago) the latter asks if Tyreese played back in the 1999-2000 season, which Tyreese specifically corrects as 1998.

-In the Christmas short special featuring Morgan and Duane, Duane's present is a scavenged Game Boy Advance. The hand-held gaming device was released in 2001 before being succeeded by the Nintendo DS in 2004. He is also seen reading a discovered Invincible comic, released in 2003.

-When |The The Prison's generator is restored and the character's discuss what movies to watch, Maggie directly names Kindergarden Cop, a 1990 film. They also refer to the format of the movies as DVD's.

-On the way to the Alexandria Safe-Zone, the group scavenge food, which Carl fondly states as Twinkies, a popular American spongecake snack.

The Zombies
The origins of the epidemic are largely unknown to the characters and the readers. Since the storyline mostly follows Rick Grimes, who was comatose at the time the outbreak started, no information of how the outbreak began has been shown. There have been several instances with the various introductions of new characters over the course of the series where their own perspectives and knowledge of the beginning of the crisis was revealed. With each new character interacting with Rick and the survivors, the reader gains a little more insight with their knowledge and memory of the origins of the apocalypse.

Early in the series it was believed that zombies were only created from other zombies biting a human. However, with the death and subsequent reanimation of Julie (who was shot and bled to death, not bitten), it was eventually revealed that any human who dies with their brain intact will reanimate as a zombie. Rick sums up that all of the survivors must already be infected, and that the bites just kill the victim unless the area is cut off and infection prevented/treated. Undead body fluids getting on a human will not turn them either, as Tyreese's reanimated daughter's blood did not convert him. The Governor's constant physical and sexual contact with his zombie daughter didn't affect him either.

The zombies of Walking Dead resemble traditional horror film zombies in that they walk and crave flesh. The zombies are slow moving but tireless, requiring no source to function. Their most dangerous weapon is their bite, which will result in a slow death, but if the location of the bite is quickly cut away, death can be prevented if wounds are treated, preventing secondary infection. The "rules" of reanimation and function remain largely similar to George A Romero's "Living Dead" Zombies, which the series got it's inspiration from.

In the first issue, Rick discovered an emaciated zombie lying nearly immobile on the side of the road, so it seems that the zombies can starve (or decay) into immobility. The only way to "kill" a zombie is to destroy the brain, the most common methods being decapitation and gunshot.

The main characters have classified the zombies into two types: "roamers" and "lurkers". "Roamers" walk while seeking new victims. Roamers are attracted to noises, and tend to walk towards any new sounds, sometimes forgetting why they are walking towards it until a new sound occurs or victims are found. As observed and explained by Abraham, the "roamers" also follow groups of "roamers" (who are heading towards noises that they may have long forgotten) out of an inborn sense of curiosity, and when enough of them gather, it creates zombie packs and herds, with as few as several to as many as hundreds of them together. The "lurker" category simply remain motionless and "play dead" until a human gets close enough to get bitten or a zombie comes too close and the "lurker" "yells" at it. The character Alice notes that she and her original group of survivors referred to the zombies as "biters," because while some do lurk and some may roam, they all will bite and to classify them into separate groups is a silly practice.

Zombies also steer clear of humans who smell like zombies, such as when Rick and Glenn cover themselves in the gore of zombie bodies and manage to escape notice from other zombies until rain washes it off. Similarly, Michonne was able to travel by shackling two zombies, whose arms and lower jaws she had removed, and walking with them.