Well, when I first saw it I thought they were making a movie adaption of Resident Evil 2, if that helps, so be prepared for some mighty spoilers.
But basically its starts off with these people going into a research facility and freeing some test monkeys. The monkeys are infected with this virus, and when they people let them out, so are all of them. The virus pretty much turns whatever it affects into not quite a zombie in the literal sense, but a person, who runs around killing other people. Like PCP I guess, but worse. Anyways, 28 days later, this dude wakes up in a hospital and is all alone. Instead of looting like I would have done, he goes around looking for people. The problem is he can't find anybody. The whole towns vacant. Then he stumbles upon a group of zombies, then two survivors save his ass, then basically the rest of the movie is them looking for more survivors.
I loved it. It wasn't what I was expecting, I mean the zombies aren't actually dead, but still it's worth checking out.