From the Game's Introduction:
Throughout the course of human myth, we have carried with us stories of mad animal-men, lycanthropes, that plague civilization, tearing through men, eating human flesh, violating women and animals, and infecting humanity itself.
Obviously, we've discovered enough about human biology to know men do not and cannot physically turn into wolves, bears, cats, or anything else. But, traditional myths about lycanthropes and modern-day psychology bear little resemblance to the Hollywood-driven "hairyman-with-claws" image that has permeated pop culture.
The term "lycanthropy" may better be replaced with therianthropy: the connection of a man's spirit or mind with the spirit or mind of one or more animals. Lycanthropes, as detailed in modern psychology, are people with mental aberrations that send them into an angered, bestial state where they behave like a frightened animal, often clawing and scratching at people who approach them.
This is not entirely different from early werewolf myths: the myths that were not folklore, but fully believed by their tellers. These stories told of men bestial in nature that would roam the night, screaming and howling and attacking others with a fevered strength.
This game is about those lycanthropes, predators on the edge of humanity.
This game is about the fears of man as he progresses into civilization, leaving the instincts that lead him there behind, that those instincts may come back, and, to put it rudely, bite him square in the ass.
Urge does not concern itself with the exact spiritual details of its characters' possession. As appropriate to your game, they may be possessed by animal spirits, in touch with their primordial roots, or dangerous aberrant psychological freaks. What they definitely are, though, are people with something dangerous and violent inside them, a predator nature that they must continually fight against in order to maintain some semblance of humanity.