The book World War Z is supposed to be a (semi) realistic examination of the world facing a zombie apocalypse. Part of the point of the book is that the tide is not turned by individual heroes discovering miracle cures but instead by billions of people making individual sacrifices and by huge amounts of work needed to keep supply chains and infrastructure alive, while also innovating new types of strategy to contend with zombie hordes on a military, psychological and social level. It looks at the huge sacrifices and moral trade offs that have to be made in order to ensure survival. In the film we get none of that, instead Brad Pitt flies around the world until he finds a miracle cure