Okay, I know I'm a bit late with this since the movie came out in June, 2013. But I just watched it this past weekend.
First off, the movie has little in common with the book except the title and zombies. And not even the same kind of zombies (the old debate about fast zombies vs. slow zombies). The full title of the book is "World War Z: An Oral History of the Zombie War". It concerns the author's journey 10-years after the zombie war to interview survivors across the US and around the world. The author is Max Brooks, son of Mel Brooks. The book was a series of personal survival stories, very harrowing and believable.
The movie is about Gerry Lane (Brad Pitt) surviving one close encounter after another with wild speeding zombies. I was somewhat amused to see that the open scene was in Philadelphia, my home town. Why Philadelphia? Why Not?
Anyway, Lane was a troubleshooter for the UN, going into tough situations and doing something, it was never clear what. But he's burned out and just wants to just spend some quality time with the family. His bad luck that zombies happened.
If you like zombie movies, the film was OK. If you like zombie movies then you're already acquired most of the suspension of disbelief that you need.
Lane sets out looking for Patient 0 with a young genius virologist, Andrew Fassbach, played by Elyes Gabel, who can currently be seen in the goofy TV show "Scorpion". Fassbach is described as the one man who can save us. Lane and Fassbach fly to South Korea where it is rumored, Patient 0 could be found. For this most important mission, by which the fate of all human kind could depend, they send a giant airplane with four guards. Fassbach and the 4 guards are killed almost as soon as the plane lands. The soldiers at the US base they land at manage to get the plane refueled so that Lane and the pilot can leave. Do any of the soldiers decide to go with them, either as protection for this important mission, or at least to maybe go somewhere safer? No.
I contend that you can suspend your disbelief about zombies in a movie like this but the behavior described in the last paragraph can not be believed.