In the film the UK has been swept by a virus 28 weeks ago. It killed all but a few, and the Americans have stepped in now that those infected with the virus (they can only live if they have people to bite apparently). The problem is that they've found some one who's immune and a "Typhoid Mary" style carrier. The problem is she infects people before they decide what to do.
The film works for a couple of reasons. Firstly it's suitably scary in all the right places. Secondly it works in similarities to Iraq, which surprised me, given that I expected it to me just a straight schlock horror film.
Incidentally the original film was written by Alex Garland (who wrote The Beach). I was in the same year at school as Alex. At the time I thought he was a complete muppet. He was always getting in trouble.
On the subject of people I went to school with, the composer Thomas Ades was in my year too. I wish I could say I knew his work, but I don't.