"The Zombie Plague, also called the Curse of Unbelief, is a deadly viral infection that was spawned by the forces of Chaos and led to the creation of hordes of infectious, undead zombies that threatened billions of people on many Imperial worlds near the Eye of Terror in Segmentum Obscurus. The first recorded incidence of the Zombie Plague in the 41st Millennium came in 757.M41 on the world of Hydra Minoris. A quarantine was imposed by the Imperial Navy, trapping 23 billion uninfected people alongside a rising tide of hungry and mindless undead.
BiologyThe Zombie Plague is a Chaos infection created through the use of Chaos sorcery blessed by Nurgle to warp a biological virus. The resulting physical symptoms spread rapidly before Abaddon's 13th Black Crusade and was heralded by the invasion of the Plague Fleets of the Chaos Champion Typhus. This led to the rise of numerous Plague Zombies on Imperial worlds and also resulted in the formation of a new branch within the Imperial Inquisition known as the Ordo Sepulturum which was specifically tasked with dealing with the Zombie Plague and similar Chaos-infused biological threats to humanity. The plague was a necrotic organism that physically degenerated those it infected, causing their skin and internal organs to rapidly rot until the moment of "death" at which time the Chaos energy within the virus reanimated the victim's corpse as a "zombie" whose only remaining drive was to feed on living flesh and so spread the infection.The Chaos energy in the Zombie Plague virus allows a small portion of the victim's life essence to be retained by their body even after physical death has occurred.
There have been numerous noted similarities between the Zombie Plague's ability to use the power of the Warp to reanimate a victim's corpse and the operation of the Golden Throne, which uses the power of the Warp to keep the Emperor's body from completely dying. Increased study of the plague by Thorian Inquisitors has led them to believe that its study can help them understand the Emperor of Mankind's current state and how he might yet be reborn. However, every attempt at creating a stable "faith virus" that would allow wounded servants of the Imperium to live on as "good undead" like the Emperor or even allow the Emperor to be reborn in an undead form has led to mutation in the sample. The zombie victims of the plague are noted to have a psychic connection created by the Chaos energy that exists within the virus and it has also been noted that psykers have the capacity to manipulate the pitiable decaying creatures that are spawned.
A common theory among the Thorian Inquisitors is that the plague is an offshoot of the technology used to create the Golden Throne by the Adeptus Mechanicus ten millennia ago and may have been created by the Dark Mechanicus tech-priests that reside within the Eye of Terror with the Chaos Space Marines."
all from warhammer 40k wiki:
also a pdf from GW to play in Apocolypse
757.M41 First recorded incidence of the Zombie Plague. Hyrda Minoris quarantined.-warhammer 40,000 5th edition rulebook