Continuity

Creator Robert Kirkman has ambiguous answers to central questions about The Walking Dead. The Walking Dead is assumed to be rich in history, but 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. 

Cause of the Zombie outbreak and government collapse

Kirkman said that going back to explain how the government originally collapsed, "...doesn't interest me, for the time being...I may change my mind eventually." As to the cause of the zombie outbreak, Kirkman wrote, "I have ideas [about the cause of the zombie plague]...but it's nothing set in stone because I never plan on writing it. So yes...I do know...kind of."

In response to a question:

"I think you should elaborate more on how people can turn into zombies without one biting you, or how this whole mess started in the first place. Was it like a plague or a rapture kind of thing?"

Kirkman responded:

"...That starts to get into the origin of all this stuff, and I think that's unimportant to the series itself. There will be smaller answers as things progress...but never will we see the whole picture."

There have been instances throughout the series where the characters theorize about the origins and cause of the undead or converse about their observations of them over time. The characters have acknowledged and discussed the lack of undead mobility and presence during the Winter months, and Eugene has specifically discussed the physical deterioration of a zombie clawing at Rosita. Most recently, Heath encountered an undead with blackened skin and a weaker will to attack.

Fear The Walking Dead will be a prequel to the television series, taking place during the start of the zombie apocalypse and showing attempts to control it. The origins of it will be explored in 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 has 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. In specific regards to the timeline, Robert Kirkman wrote, "The Walking Dead takes place in our world, as if this stuff had started happening in October 2003 and continued from there. Now, (Issue 38) in the book...it's still only like... June 2004 at this point (If you're keeping track)." "[The Walking Dead] is set in modern times...but the book started in 2003 and only a year has passed in the book. But that doesn't mean it's 2004 in the book...maybe it's 2009...who knows...who really cares? I don't want to be specific." The statement regarding the amount of time that has passed is however contradicted in the novel Fall of the Governor, which states that it has been over two years since the dead started walking. And the comic's more than one-hundred issue run has had the characters state that the time passed from the beginning to the latest issue has been over two years. However, nothing has established the exact point of divergence within the Walking Dead universe and real world history, or if the Walking Dead universe has been following an alternate history of its own before the epidemic. At one point during the series, Andrea had attempted following a self-made calendar which was implied to have been dropped, a point of maturity for the survivors.

Direct references to specific things such as particular areas, names, technology or companies and brands are kept to a minimum. However, several instances throughout the series, in dialogue and in passing, reveal parts of the characters' surroundings and past. This gives some insight into at least a general guess as to the era of the epidemic.

However, recently in an episode of the spinoff, Fear The Walking Dead, the fresh graves of man's family are shown to all have an end date of 2010, and not yet a year had passed in the show. This is mostly likely the year the outbreak started.

• 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) she asks whether 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 gift 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 prison's generator is restored and the characters discuss what movies to watch, Maggie directly names Kindergarten Cop, a 1990 American film. They also refer to the format of the movies as DVDs.

• While the group check their weapon inventory, Rick says it's okay to leave the generator on a little while longer because everybody else was watching and enjoying Turner and Hooch, a 1989 American film.

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

• When speaking with Dexter, Rick mentions that it had taken a while for the group to begin calling the walkers zombies. The Walking Dead's form of zombies hadn't been created until 1968's George Romero's, Night of the Living Dead.
• Glenn states in the episode, "Pretty Much Dead Already", that he has played Portal, which was a game released in 2007

• In Fear The Walking Dead, Travis and Chris and a group they were traveling with encounter graves at a farmhouse. All of the graves are dated with 2010 as the death date. His entire family died in 2010 and not even a year had passed in the show. It is most likely the zombie outbreak started in 2010.

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 on 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 background 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 humans. 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 or treated. Undead body fluids getting on a human will not turn them either, as Tyreese's reanimated daughter's blood did not convert him. Cannibalistically eating another human, as confirmed in the video game episode Starved For Help, also does not seem to cause turning. The Governor's constant physical and sexual contact with his zombie niece 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 of energy 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 a secondary infection. The "rules" of reanimation and function remain largely similar to George A. Romero's "Living Dead" Zombies, from which the series got its inspiration.

In the first issue, Rick discovered an emaciated zombie lying nearly immobile on the side of the road, so it seems that 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.