User blog comment:BannedLagiacrus/Monster Ecological Info/@comment-25119729-20141015154848/@comment-6050823-20141016020540

Actually, there is no Extreme Virus, that is just the Frenzy Virus. The Extreme Monsters are just monsters that have overcome the effects of the Frenzy Virus and have also gotten immensely stronger from the Frenzy Virus(Explaining why they are immune to all sorts of things)similar to how hunters can overcome the effects of it. Unlike the hunters, the Extreme Monsters can spread around the Frenzy Virus and transmit it to other monsters just like both the Magalas and the Frenzied Monsters.

Also, the virus was caused after the death of the Shagaru Magara due to after dying the virus it produced spread much farther due to Shagaru Magara not controlling it and ended up infecting other far away areas.