User blog comment:Kogath/Monster Hunter 4 Ultimate for the Nintendo 3DS/@comment-21371438-20141016212052/@comment-6050823-20141017011440

Here is the true explanation behind the Extreme Monsters, which comes from MH4G; The Extreme Monsters are just individuals 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. What sets the Extreme Monsters apart from the Frenzied Monsters is that the Extreme Monsters actually survived without dying from the effects of the Frenzy Virus while most other monsters(the Frenzied Monsters)usually would've die shortly after being infected.

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.