At the end of the book (and the movie as well) the vampire bites Bella, meaning to kill her and lure the Cullens into fighting him. However, Edward arrives in time to prevent James from killing Bella; instead he just manages to bite her hand. According to the book, when a vampire bites but does not drink all of a person's blood, then the venom from their bite spreads throughout the victim's body and turns them into a vampire. This is a painful process, and it burns like fire through the person's veins. When James bites her, this process begins to happen to Bella. In order to make it stop, Edward must suck the venom from James' bite out, and yet stop in time before he drinks too much of Bella's blood which would kill her. Somehow Edward finds the strength to stop, and Bella survives and remains human.