SentinelSevn
Professional Coffee Addict
It could be that they discovered a new assimilating kind of virus that mutates faster than we can study it that by the time mecha were developed to finally counter these monsters, they'd spiralled out of control already. That would also require the mecha possessing "something" that can kill these mutants that normal conventional arms do not possess hereby throwing all pretense of "realism" out of the window completely. Mecha can be good even if it's not realistic. There's a reason why 80's-90's Super Robots are so loved until today.
So rather than some sort of regenerative research, its a recently discovered virus? Or maybe the discovered virus has a trait akin to cell division, which was what brought upon the idea of regeneration. Still keeping along the lines of spiraling out of control though, but they would at least have had some pretense or motivation to study it, no?
As for defeating them, since I had tried to link the mutation to the pituitary glands, I was going to go for that being the weakness.