Sans and Papyrus have a duty within the AU to deal with monsters who have turned into "zombies" due to accidental deaths. So, does their training also include content about monster weaknesses and how to completely eliminate them?
The Dead Cell is mutated of Fallen Cell. That's what Abyss thinks. Again, he never had the time to examine them to truly understand where this Dead Cell came from and why.
The term "zombies" used by Abyss to refer to the Dead Cell that act like undead monster.
The Fallen Cell is a mutation that occurs when the host is on the verge of dying.
Gaster and Sans was the one who eliminated the Fallen Cells. Papyrus on the other hand hesitated at first, because he thought the Fallen Cell are still monsterkind with different form and try to help them. Until he realizes that Fallen Cell has no consciousness and is no longer sentient being. A creature that seeks only to attack and devour anyone who crosses its path. His recklessness almost cost his family their life.
The reason for is that the few Monsterkind are injected by Cell during their fall down, that is, their coma, and they wake up without any problem. No amalgamation exists, with the exception of Memory Head.
Sans and Papyrus are born with Cell, which is why they can transform into powerful beings and return to normal.
Something you may notice is , Cell begins to infect other monsters that were not previously injected by Cell. Like Toriel, for example, who finds it strange that her fire magic is starting to change color and become stronger.
Well, you just have to find out in the comic and be patient.
