It has been announced today that War Z will now be known as Infestation: Survivor Stories. In a statement on the community forums, community manager Kewk has said that "This change has come about primarily as a result of some confusion and trademark issues with a similarly titled property."
The only thing I can think of there is either Day Z or the new World War Z film that has just been released, not entirely sure if there was any confusion mind you. Kewk continued by saying;
"While we were reluctant to rename the game so long after launch, especially with nearly 1 million registered players, it was ultimately decided to be in the best interest of our existing community as well as future players in order to eliminate confusion."
As far as we know nothing has changed in the game at all and it is simply a re-branding, meaning that the game is still as bad as it was when it released.
Source: PC Gamer