Do you think a zombie attack could ever happen?

I don't mean zombies like the living dead, i mean a zombie more like a violent disease. We saw how quickly swine flu spread and how we couldn't stop it, so say a virus that's maybe 20 times as strong as that? It would spread so quickly, and we live in a society totally unprepared for such a catastrophe. And it doesn't have to be spread by biting, maybe just airborne? It's like "I Am Legend" where only a certain type of people caught the disease, and those who caught it fed off those who didn't. So do you think it could really happen? Thanks for your opinion :)