28 Days Later is Zombie Horror and if you try to call it anything you're just nitpicking.
This reminds me of all the jokes on sitcoms about Star Trek actors facing their nerdy fans that like to point out and probe them on some stupid little technicality in an episode ten years ago.
Technically the virus has made the city have bloodlust without killing them first but who really cares? There's still a city full of people that arent curable and are trying to eat ya.
The main genre is horror. The sub genre is zombie.
I can't even believe there are SO MANY people that would sit and combat such a petty fact. It's clearly a zombie movie and the director has even said so in interviews.
EDIT: ADDED LINK TO ONE OF INTERVIEWS
http://www.indiewire.com/people/people_030627boyle.html