The upcoming Undead Nightmare expansion for Red Dead Redemption is both a rather difficult and easy concept to grasp. On the one hand, it's hard to understand how or why this zombie plague is striking the American Wild West. (Rockstar still wouldn't talk about story details.) On the other hand, it's an easy excuse to introduce new enemy types into Rockstar's open world. Undead Nightmare adds a lot of things to shoot at, and looks to be a meaty extension of the game's single-player experience for a rather reasonable $10 price.
Perhaps as an admission of its incompatibility with the canon of the world, Undead Nightmare is a standalone experience that takes place between the end of Marston's quest to find his family, and the game's final battle. Nothing is carried over from the main game, with the exception of any new weapons unlocked by the game's other DLC. The goal of this quest is to rid the country of a zombie infestation, a journey that will take a player anywhere from 5-12 hours, depending on their skill and propensity to complete side quests. Rockstar wouldn't reveal exactly how much of the map would be infected, but they did tease that a new area would be added to the world via this expansion.
The cure to the undead plague appears to be rather simple. At cemeteries, there will be a collection of coffins you'll have to burn. Of course, this is easier said than done. [Update: Rockstar offered clarification, adding that "clearing the graveyard is just a small piece of the narrative and it is one of the many theories people in RDR have as to the cause of the plague. But it's definitely not the entire plot of the game."] Burning one coffin will trigger a massive influx of enemies, as they rise from the ground. As you burn the coffins and fight the swarm, you'll eventually encounter a boss zombie. Take him down, and you'll rid the area of the zombie menace, clearing the skies in the immediate vicinity whilst lowering the frequency of random "ambient" zombie attacks.