I had always assumed that they just incorporated any and all organic matter into their mass, under the overarching dominance of the Rash. That's certainly how I've been writing them in fics. Maybe the immune people and animals are the bits that retain enough mind to scream?
Yes that's what I've thought all along. The way immune people act around Rash beasts is still in fear. As far as we have seen the Rash doesn't eat whatever it catches, it just infects it. So my thinking is that the Rash feeds off of healthy host bodies, as does any normal disease, and when it catches an immune person whose body it cannot infect, it just holds onto it anyway. It's the Rash. It likes claiming things for its own and warping them beyond recognition.
Of course I assume an immune person could crawl out if they had a knife, determination and a very loud voice to call for help. I've actually been entertaining an idea for a while where a troll happens upon the Hotakainens in the woods, Onni splits its rib-cage open and out plops Lalli's father.
