Rellik7778 was right; this film wasn't horribly scary.
Now that we're past that, I would like to start out by saying that this is a British film about a virus that breaks out when animal rights activists release a bunch of infected monkeys. The monkeys trigger an outbreak of a disease called 'Rage'. When Rage makes contact with any bodily fluid (blood, saliva, etc.), it takes over the body within twenty seconds and turns you into a mindless eating machine. Soon, virtually all of London is consumed by this plague, and the main character of our story is left in a dire situation.
You see, Jim (Cillian Murphy) awoke from a coma twenty-eight days after Rage was unleashed and found a deserted city. Attacked by the infected, rescued by survivors, Jim doesn't exactly like his new surroundings. He and Selena (Naomie Harris) seem to be the last humans still alive... or are they? Will they be able to find a place in this doomed earth? Other important characters are played by Megan Burns, Christopher Eccleston, and Brendan Gleeson.
All of the actors are amazing, and I was really pulled in by the realism. 28 Days Later is a great adrenaline rush, worth a rent with your friends.