No evidence supports the belief that werewolves are real. Legends about werewolves and other shape-shifters are common in many different parts of the world. However, there is no physical evidence that a human can actually change into a wolf.
Rabid wolves are one possible source for the legend of the werewolf. In humans, the symptoms of rabies include aggressive behavior. People who contract rabies from a wolf bite may be seen as having taken on the characteristics of a wolf.
Some genetic disorders also play a role in these legends. For example, congenital generalized hypertrichosis involves excessive hair growth on the face and body that gives the afflicted person the appearance of an animal.