I remember watching the beginning of this movie and thinking that I was in for a snoozer.  When the animal activists broke into the lab and freed the animals, I thought that maybe this had taken some moral stance on animal rights or something along those lines.  Don't get me wrong, I support animal rights, but have no desire to bring the issue into horror films.  
If you have already seen this film, you likely know by now that it would be very difficult to dislike it.  I am a huge fan of zombie films, and this one takes the concept into overdrive.   The overall product is so good, that I have heard praise from a number of people that generally don't like horror.
Some lab monkeys have been infected by a "rage virus" by a group of scientists.  This is probably not a huge deal, unless a group of animal activists free the monkeys and they spread the virus to humans.  That is, of course, excatly what happens.  Sort of a novel idea, and it crosses into a zombie type of film in a hurry.
With the virus spreading very quickly and the city being overtaken, Jim (Cillian Murphy) wakes up in a hospital unaware of what has happened.  He ventures out into the city to discover the streets are bare.  He meets up with a woman that has not been infected and they attempt to survive the ensuing slaughter.  As the film goes on, they meet a father and daughter who work with them.
There are some great scenes of Jim and the team fending off the infected.  One of the great things about this film, one item that differs greatly from a typical movie of this type, is that the infected turn very quickly and are athletic and extremely fast moving.  This really helped the film from an action standpoint as the result is a very fast pace that is remarkably engaging to the viewer.  There is not a whole lot of waiting around and devloping plans as the infected are extremely aggressive and move quickly to attack.  
Danny Boyle did a fantastic job with this film.  The direction, dialogue, action, sets, gore and acting are all well above average. 
Eventually, Jim and Selena run into the armed forces and are taken in by them.  Unfortunately, they are corrupt and have no desire to do anything but mistreat them.  The troops even have a "zombie on a leash".  One of their own infected is captive by means of a large chain. 
In the end, the pair escpes the armed forces also and the end leads you to belive that they live happily ever after.  However, the saga is not over quite yet.