World War Z is a zombie novel by Max Brooks, and the followup to his hugely successful satirical Zombie Survival Guide. WWZ posits a world in which a viral outbreak caused a worldwide pandemic of "zombies," revived dead humans who are motivated by a mindless hunger.
The book is written as the history of the pandemic as recounted by various front-line survivors to a journalist. In this sense, the book acts as an alternate history to modern history, since all events are presumed to be true and there is no "fourth wall" to break; Brooks never speaks outside of the narrative, and except for our knowledge that there are no zombies, there is no reason to read the book as anything other than a factual account.
It can be assumed that the publication of the book is in the future, after the actual events detailed within. While it does not attempt to take a real-world event and extrapolate a possible alternate future, WWZ falls at least partially into that category:
According to [author and editor] Steven H. Silver, alternate history requires three things: 1) the story must have a point of divergence from the history of our world prior to the time at which the author is writing, 2) a change that would alter history as it is known, and 3) an examination of the ramifications of that change.
(Wikipedia, "Alternate History," Definition)
Because WWZ does not explicitly state a timeline, it is strictly speaking a "what-if" type of story. However, WWZ instead speculates that many disasters and conflicts in history were in fact zombie outbreaks which were covered up. This covers the first requirement, and the book itself entirely comprises the last two, since a zombie apocalypse would obviously "alter history as it is known."
This record of the greatest conflict in human history owes its genesis to a much smaller, much more personal conflict between me and the chairperson of the United Nation's Postwar Commission Report... it came as a shock when I found almost half of [my] work deleted from the report's final edition.
(Brooks, World War Z, Google Books)
By styling the book as a "recorded history" of personal experience, Brooks avoids the standard "zombie novel" form and instead creates a plausible alternate history of world conflict.