You must understand two things about World War Z the movie.
One, never forget that the only thing the film has in common with World War Z the novel is the title. If you read the novel you must remember this point or massive disappointment will smack you upside the head.
Second, make sure to turn off brain upon entering the theater. Do this and you'll enjoy the film. But if you accidently fall into thinking, be warned—logical loops and bullshit will erupt from all frames of the film, and you'll emerge from the theater arguing that this couldn't actually happen. Not because of the zombies, mind you, which are now acceptable SF tropes despite having no means of existing in the physical, entropy-dictated world.
No, you'll go crazy trying to explain the other silliness in World War Z.
Because—"Argh! here be spoilers" (said in pirate talk for no particular reason)—the world of World War Z is saved from zombie infestation by infecting humanity with a deadly disease. Wait a moment and let that tickle through your brain. You see, Brad Pitt discovers, at immense risk to himself, that zombies won't attack someone who is already ill with a fatal disease. Because, you see, those zombies want to spread their infection and infecting someone who is already going to die won't spread it.
Get that? In a movie where zombie infections spread person-to-person in less than 10 seconds, zombies don't want to infect a man with cancer who will die in a year or two.
Brain. Exploding. Which might not be a bad thing, if I was a zombie you needed to kill.
But ignore that logical loop. Instead, think on how Brad Pitt saves humanity. By infecting himself with a disease which will kill him. Meaning the zombies won't kill him. But as the other characters say, "He's a dead man anyway."
Except he's reunited with his family in the end. So obviously that fatal disease wasn't overly fatal, or they could cure it, which makes you wonder why the zombies couldn't sense this and decide, heck, he'll live longer than the 10 seconds we need to turn him into a rapid zombie vector.
Argh!
Still, World War Z is a fun movie. Just don't over-think it. Or think at all during the film.