User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-3277384-20141109160954/@comment-6050823-20141109171956

All Extreme Monsters, similar to the Revolving Dragons(Magalas), can spread the virus around and infect other monsters. Extreme Monsters are individuals that were able to overcome the effects of the Frenzy Virus and have also developed a relationship with it granting them: Immunity to Elements/Aliments/Traps and increased physical strength/speed/defense. From the Extreme Monsters, the Guild developed a way to fight them and hunt them, the Purification Stones. They were developed for hunters to fight off the Frenzy Virus and to temporarily cleanse the virus from both the Infected Monster's and Extreme Monster's system, allowing hunters to hunt them with more ease and less of a death struggle.