A werewolf is a human being who, upon the complete rising of the full moon, becomes a fearsome and deadly near-wolf. ma sói can be easily distinguished from regular Người sói bởi their shorter snout, thêm human-like eyes, the tufted tail, and their mindless hunting of humans whilst in chó sói, sói 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 bởi infection with lycanthropy, also known as werewolfry. A werewolf cannot chose whether hoặc not to transform and will no longer remember who they are and would kill even their best friend được trao 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 niêm phong, con dấu 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 bởi Damocles, allows the werewolf to keep their human mind during transformation.
Lycanthropy is a magical illness known to be spread bởi 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 bởi 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 hoặc scratch obtained from a werewolf, whether in human hoặc animal form, will leave permanent scars.
ma sói 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 ma sói 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 chó sói, sói cubs - actual chó sói, sói cubs - who grow to become very beautiful Người sói and can only be distinguished from true Người sói bởi their near-human intelligence.

Thus, rumours of ma sói living in the Forbidden Forest in the grounds at Hogwarts lâu đài 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.