The Marvel Zombies universe is an alternate reality, designated Earth-2149, in which most of earths heroes have been infected with a virus that has turned them into zombies. While the true origin of the infection to Sentry is still unknown, it is a paradoxical loop involving a Sentry from yet another alternate reality, (Who in concept arts looks strikingly similar to DC's Superman, down to chest logo and color scheme) that is responsible for its existence in the main series seen in the Marvel Zombies. Marvel Zombies 5 outlined that some of these universes have a unique variation that differs from the established virus. Any connection between the version seen the main series and the alternate versions seen in other comics is unknown.
Examples that have been shown in the alternate universes are Marvel Zombies Supreme and Marvel Zombies Destroy. In Supreme the zombified Squadron Supreme is created when scientists graft the original team's DNA to human corpses and then uses space radiation to revive them. In Destory Loki plays a trick on Thor by infecting humanity so that Asgardians could rule.
See also Marvel Zombies team
The origin of the Marvel Zombie Universe was created by Mark Millar and Greg Land who wrote two story arcs of Ultimate Fantastic Four titled "Crossover" and "Frightful". These story arcs were inspiration for the Marvel Zombies limited series by Robert Kirkman and Sean Phillips.
For more information on the Story Arc see Marvel Zombies
Main Series (Earth-2149)
Marvel Zombies: Days Dead
Prequel that shows the zombie infection spreading throughout the world, while uninfected superheroes try to devise a plan to save the human race inside the S.H.I.E.L.D. helicarrier.
Marvel Zombies: Evil Evolution
A One-Shot crossover between the Zombies and the Marvel Apes Universes (Earth-8101 or 95121). It fills in additional information about the origin of the zombie plague and is set between Dead Days and Marvel Apes Crossovers. It is also implied that the dimensional portal Reed Richards was testing had brought Zombie Sentry to this Earth.
Ultimate Fantastic Four
Crossover (issues #21–23)
Reed Richards (Earth-1610) receives contact from another version of himself from an alternate dimension. After crossing over to the other dimension, Reed discovers he's been tricked and the entire world is infested with zombies who wanted to use the Ultimate Marvel Universe as a food source. The Fantastic Four of the Zombie Universe failed to capture Reed and crossed over to the Ultimate universe through the dimensional portal. With help from Magneto, Reed escapes the zombie universe and prevents more zombies from entering the Ultimate Universe.
After zombie superheroes eat the human race to extinction, Galactus prepares to eat the Earth. Galactus sends his herald the Silver Surfer to Earth only to be eaten by Captain America, Giant Man, the Hulk, Iron Man, Luke Cage and Spider-Man. They then take the attack to Galactus. Now infused with the Power Cosmic, Giant Man, the Hulk, Iron Man, Luke Cage, and Spider-Man, travel the universe as the to search for more sources of food.
Ultimate Fantastic Four
Frightful (issues #30–32)
The Zombie Fantastic Four attempt to break free from their confinement. Also Ultimate Johnny Storm is infected with a demon from the Negative Zone, and the Ultimate Reed Richards performs a mind-swap with the Ultimate Doctor Doom. When the zombies break free, Reed (in Dr. Doom's body) becomes the demon's new host and uses its power to defeat their zombie conterparts. Dr. Doom reverses the procedure and swaps their minds back to their own bodies, he then sacrifices himself by going through the portal to destroy it and trapping himself in the zombie universe.
The current Fantastic Four (Earth-616) of Black Panther, Storm, Human Torch and Thing, are teleported by King Solomon's Frogs to the Marvel Zombies Universe where they encounter the Marvel Zombies on a Skrull planet.
Marvel Zombies 2
After they had devoured all life in the universe over the last forty years, the Zombie Heroes return to Earth. They have the idea to access other dimensions for food. When they arrive on Earth, they face off against New Wakanda, a colony of humans who survived the zombie onslaught.
Marvel Zombies Return
After the events in Marvel Zombies 2, zombie Spider-Man, Giant-Man and Wolverine are teleported to a different universe. The series deals with the ramifications of their arrival on Earth-Z. Again with the lack of food they must obtain cross-universal teleporters.
The series ends with the Watcher of this universe sending the infected Sentry back in time to the moment when the infection emerged on Earth-2149. This in turn contained the zombie virus to only in those two universes creating a contained paradoxical loop.
A.R.M.O.R. & Guardsmen (Earth-616)
Marvel Zombies 3
When the zombies invade the Marvel Universe, Machine Man, Ultron and Jocasta go to the Marvel Zombies universe to retrieve a blood sample from an uninfected human. Unfortunately zombie Morbius (Earth-2149) had plans of his own, and to use the blood to infect the Marvel Universe.
Marvel Zombies 4
The Midnight Sons, working for A.R.M.O.R., hunt zombies who have escaped to different universes. Elsewhere Dormammu commands The Hood to capture the virus for his own ends.
Marvel Zombies 5 (Earths-483, 691, 8410 & ?)
After the events in Amazing Spider-man #622, A.R.M.O.R. discovers that several alternate realities afflicted with a variation of the zombie infection will temporarily merge during a cross-temporal planetstorm. Morbius orders fellow A.R.M.O.R. agents, Machine Man and Howard the Duck to retrieve samples of the infection from each reality so that he can find a cure for his infected friend, Jack Russell.
Marvel Zombies Supreme
Responding to a mysterious emergency call, Jill Harper of the Guardsmen receives an emergency call and takes her special-ops unit into a top secret underground research facility. They discover the experiments of the geneticist has created a new zombie strain. They used Squadron Supreme (Earth-712) to revive human corpses, which reduced them to cannibalistic murderers with a hunger for human flesh. Harper and her team fight against to stop the zombie menace, before it spreads
Marvel Zombies Destory
A.R.M.O.R. is called upon to rescue a reality where Zombie Nazis (Earth-12591) won WWII. They are now using Asgaurd magic to teleport themselves to every other universe. Howard the Duck enlists Dum Dum Dugan and a squadron of fighters known as Ducky's Dozen to stop them.
Marvel Zombies vs. Army of Darkness (Earth-818793 "Army of Darkness")
Set before, and during, Marvel Zombies: Dead Days. Ash Williams is transported to the Marvel Zombies universe in Army of Darkness issue #13, and must fight the zombies while searching for a way to escape the universe after he finds the Necronomicon.
Deadpool: Merc with a Mouth
Deadpool teams up with his zombified head (Headpool) after discovering it in the Savage Land. He then goes to Headpool's Universe (Earth-2149) to return him.
Deadpool is still teamed up with Headpool for this mini-series. None of it takes place in the Marvel Zombie Universe.
Spider-Ham (Earth-7044) is transported to the Zombie Universe and infected becoming Undead Ham. He is killed by Ultron.
A zombiefied Wolverine (Earth-6195), appears alongside other Wolverines on the All-Wolverine Exiles team.
Marvel Apes (Earth-8101 or 95121)
Not directly related to Marvel Zombies, but the events that occur are resolved in Evil Evolution.
See also Marvel Apes: Speedball Special
A glimpse of an unidentified zombie is seen.
See also Marvel Apes: The Amazing Spider Monkey
Red Ghost and Speedball are running away from zombie versions of Giant Man, the Wasp, Iron Man, Wolverine, and others after they came to the Ape Universe.
See also Marvel Apes: Grunt Line
Ape X, The Gibbon, Jane Potter, Gorilla Girl, and the Red Ghost's apes in the Marvel Zombies Universe.
See also Marvel Apes: Prime Eight
Iron Mandrill and other heroes receive a vision about the Marvel Zombies. The 7 most powerful Apes gather and have to work together with Dr. Doom and a Nick Fury 2099. They realize that the zombies have already arrived and one of the apes will be infected.
Marvel Zombie Christmas Carol (Earth-?)
Retelling of Charles Dickens Christmas classic with Zombies.
Marvel Zombie Halloween
Kitty Pryde tells her son Piotr Rasputin about Halloween. When he goes Trick-or-Treating he is attacked by zombie versions of Alex Power, Darkhawk, Karolina Dean, Mettle, Squirrel Girl and has to be saved by Mephisto.