While apocalyptic and post-apocalyptic fiction have been established subgenres of both speculative and conventional literature for decades -- the Roland Emmerich big-screen cheesefest 2012 and Cormac McCarthy's critically acclaimed book-cum-movie The Road being but two of the most recent examples -- the zombie apocalypse is a genre, a setting, and pop cultural movement unto itself, particularly on film. Put another way, you can be a fan of zombie fiction and/or zombie flicks without being a fan of any other genre or subgenre of speculative fiction.
We owe much of this standalone zombie popularity to George Romero and his schlocky but commentary-ridden Living Dead movie series, which has enticed devoted fans for decades. Zombie apocalypse fandom is growing steadily more mainstream, with no better example of the phenomenon than the near-ubiquity of zombie walks -- wherein dozens to hundreds of zombie cosplayers shamble through a public area en masse -- across the United States each year.
I bring it up because: First and foremost, Robert Kirkman's seminal zombie apocalypse comic book series The Walking Dead has been recently placed into production as a television series by AMC, with fans clamoring for a Mad Men-quality serious treatment of the highly thoughtful and successful zombie franchise.
Secondly, because some oddball Canadian mathematicians have modeled the zombie apocalypse and calculated exactly how small the time-window for containing such an outbreak would really be. (Verdict: We're hosed.)
Thirdly, because zombie fiction has begun to consume the classics of literature, as is the case with Pride and Prejudice and Zombies, a retelling of the Jane Austen novel with an account of a Victorian zombie outbreak integrated with the original text. (It can't be any more painful than Clueless.)