So, Slayer, your theory is basically John Carpenter's Thing, only with dead flesh rather than any flesh?
Theory, yes. And it's a odd one at that. Who knows, the Undead Parasite(or parasites) could be common in the Abyss(note, when you travel to the Abyss in DoS, there are a lot of areas that are full meaty, like your are within a living organism). Different realms, different rules. It might be the equivalent of War of the Worlds, where the Martians are uber powerful, and nearly wipe us out, but are weak against our diseases(yet to us, a common cold isn't that deadly). Kryptonite harms Superman, but it isn't as deadly to other people. I think, where ever these Undead Parasites originate from, perhaps they aren't as harmful to, say, demons. They are probably like fleas or Athlete's Foot. But, crossing over, coming in contact with the flesh of dead human bodies, the parasites go out of control, gorge themselves and assimilate the flesh, mutating into more complex and grotesque forms.
I really DO love the idea of the Undead Parasite. Something otherworldly(origins in the supernatural), but also somewhat sci-fi. Reminds me of that whole scene in Poltergeist when that portal to the netherworld tries to "devour" the kids in their room, literally becoming a giant meaty gullet, trying to suck them in:
It's an interesting way to look at the supernatural. Even, in that movie, when they(spoiler to those who haven't seen it)
... pull Carol Anne out of the netherworld with the tether, her and her mother are completely covered with slime and fleshy placenta-like meat.