In the movie The Quarantine it was a normal day for David and Will, then all of a sudden there's a big explosion. The explosion made the whole school super ill forcing the military to go in and close all openings letting the students outside. Reason behind this was because letting students out would immediately make the outsiders sick. The disease would only last until you graduated and then would disappear and be gone. Many of the students started forming their own little groups leaving David and Will alone together.
In the book it didn't tell all the little things that happened in the movie. The movie will give you a lot more details and a better pictures and images in your head. The book will explain some of the things the movie won't have or what they cut out of the film. The book will describe and tell the bigger story. I recommend reading and watching both of them just to get the full story.