A werewolf is a human being who, upon the complete rising of the full moon, becomes a fearsome and deadly near-wolf. Werewolves can be easily distinguished from regular wolves by their shorter snout, more human-like eyes, the tufted tail, and their mindless hunting of humans whilst in wolf form. At all other times, they appear as normal humans, although they will gain a pallor as the moon approaches and then wanes. This condition is caused by infection with lycanthropy, also known as werewolfry. A werewolf cannot chose whether or not to transform and will no longer remember who they are and would kill even their best friend given the opportunity once transformed. Despite this, they are able to recall everything they have experienced throughout their transformation upon reverting to their human form. A mixture of powdered silver and dittany applied to a fresh bite will seal the wound and allow the victim to live on as a werewolf, although tragic tales are told of witches and wizards begging for death rather than becoming werewolves. The Wolfsbane Potion, invented by Damocles, allows the werewolf to keep their human mind during transformation.
Lycanthropy is a magical illness known to be spread by contact between saliva and blood; thus, when a transformed werewolf bites a human, the bitten will become a werewolf themselves. Most Muggles, however, will die from the extent of their injuries in the instance of a werewolf attack as noted by Professor Marlowe Forfang. If a werewolf is in human form and bites the victim, they will merely gain lupine tendencies such as a fondness for rare meat. Any bite or scratch obtained from a werewolf, whether in human or animal form, will leave permanent scars.
Werewolves cannot pass on lycanthropy to their children, and so if having a child with a human partner no lupine tendencies will occur, as noted with Teddy Lupin, son of Remus and Nymphadora Tonks.

If two werewolves mate at the full moon, in their animal forms, something very strange happens. The result of their mating, which has only ever occurred twice throughout history, has been a pack of wolf cubs - actual wolf cubs - who grow to become very beautiful wolves and can only be distinguished from true wolves by their near-human intelligence.

Thus, rumours of werewolves living in the Forbidden Forest in the grounds at Hogwarts Castle are actually true. With the kind permission of Albus Dumbledore, Headmaster of the School, the pack was released into the woods and have lived there ever since. Teachers have never tried to dispel these rumours because they felt that keeping students out of the forest was highly desirable.