The Werewolf Genocide was an attempt by the Volturi to wipe out the Children of the Moon, started by Caius, after he was nearly killed by one and the fact that a werewolf's bite is 100% fatal to a vampire. Caius became terrified of them all, and convinced the rest of the Volturi to help him drive them into extinction. Very few werewolves escaped from this purge and the remaining ones formed packs in order to survive. The genocide happened a long time before modern times and the werewolves that survived were so few that even a majority of vampires today believe them to be a mere legend,
The actual events are largely unrecorded, but the Volturi nearly succeeded in making the werewolves extinct in Europe and Asia. It is unknown if they are found anywhere else, but it is implied that at least a small number of them still exist since James, Victoria and Laurent were hunting one in Siberia.