The myths and stories surrounding werewolves originated in Europe. Many of today's ideas seem to be heavily influenced by German Paganism.
Belief in werewolves seems to be non-existent before the 14th century. Werewolf superstitions seem to have arose along side the rising popularity of Christianity. Once Christianity became the norm, transforming into a wolf - or other beast - was seen as a Pagan belief and thus associated with the Devil.
Werewolves also have a strong tie to the European witch trials, particularly during the 16th and 17th centuries. Once the witch trials subsided, werewolves became extremely popular within folklore and helped bolster the Gothic horror genre.
There have also been medical explanations given for the belief in werewolves. I have found four medical conditions associated with werewolves:
- Congenital Porphyria (symptoms are photosensitivity, reddish teeth and psychosis)
- Hypertrichosis (excessive hair growth - hereditary condition)
- Downs Syndrome (I didn't find much info here, just that some scholars have linked the two together)
- Rabies (remarkable similarities - supports idea of being bitten by a wolf and becoming a werewolf)