A person becomes a werewolf, when bitten by a werewolf in wolf-form. Once this happens, the person must learn to manage the condition. Potionmaker Damocles Belby developed a draught called Wolfsbane Potion that controls some of the effects of the condition; by allowing the sufferer to maintain their human mind in wolf form, it prevents them from harming others. Nothing discovered in the wizarding world can completely cure a werewolf. Once in a while, this condition (or disease) can be passed down through parentage.
There are only three known werewolves in the Harry Potter series: Remus Lupin, an unnamed character who was in the same ward as Arthur Weasley in St Mungo's Hospital for Magical Maladies and Injuries, and Fenrir Greyback, a supporter of Voldemort, who bit a young Lupin. In Harry Potter and the Half-Blood Prince, Greyback attacks and kills a five-year old boy, the younger brother of two Hogwarts students. Later in the book, he attacks Bill Weasley. However, since Greyback was in human form at the time, Bill did not become a complete werewolf, but did gain some wolfish features, such as favouring very rare meat. It is also known that werewolf traits are not necessarily transferred to offspring, as seen in Ted Remus Lupin, the only child of Remus Lupin and Nymphadora Tonks, who was not born as a werewolf.
Gilderoy Lockhart claims to use the Homorphorus charm to transform "the Wagga Wagga werewolf" back into a human in his book. However, given his limited credibility (as he writes a severely altered account of other people's doings to insert himself in their role), this should be doubted. In addition, it is not clear whether this is touted as a permanent cure or whether it nullifies the Wolf transformation for the remainder of this full moon only.