'World War Z' was a great novel. Another one that comes to mind is "Cell" by Stephen King. Not really the traditional zombie stories, but both had semi-plausible explanations for their respective zombie plagues.
I would absolutely give it a go! I've been reading a lot of zombie YA at the moment. Just a couple weeks ago, my sister said I should write a zombie YA. I decided against it, but that was what spurred me to go find some fun zombie books to read. I'd recommend that you get your hands on zombie books and see what you like, what you think is believable, what you think others will find to be believable. If you're curious, the two books I read are The Enemy by Charlie Higson and The Forest of Hands and Teeth by Carrie Ryan. I think both are good examples of serious zombie books, even though they are YA. Both these books are 1st books in trilogies. I liked the zombies in these books because they were relatively slow and stupid, but the greatest danger is that they come for you in large numbers and they keep coming. Totally give it a go! And let us know if you do!
I just got done reading this thread, and there was an advertisement at the bottom of the page for Halloween costumes. One of them was a zombie costume. *Queue spooky music*