Three possibilities,  none of which have been confirmed by production. Intentionally vague ending to make the audience think.

 1. Lycanthropy is presented as an infectious diseases passed through bodily fluids. Will and Stuart got it through saliva to blood contact. There is no reason to assume it can pass through saliva to saliva or blood to blood or sexual fluid contacts. At no point did they imply she was bitten or scratched though, so the sexual contact is the only logical reason.


 2. As the expert on werewolves says earlier, some people have a wolf analog inside, and their passion can bring it out. Laura's connection to Will and the events could have caused her to manifest the demon wolf inside her. A mystical reason.


 3. My personal interpretation of the events: Laura was compelled by the events, her feelings towards Will, seeing what Will was to be, to cause herself to be infected. She infected herself with Stuarts or Wills blood after Will left. She willingly tried to become the wolf. Much like Will slowly came to accept the wolf and went off to become the wolf, it's a symbolic and thematic ending. Having Laura do the same explains the final scene showing Will's full transformation imposed over her starting her transformation.