the "cells" are losing their connection to each other, causing parts to fall of if strained. there's a similar disease/syndrome in humans, don't remember the name.
Remember that this is created with convenient science. It wouldn't mutate, and if it does, only in a convenient way such as causing even longer suffering for yukkuri.
Maybe one day, all disease will be gone thanks to yukkuri because human disease mutate to target yukkuri as dictated by nature law :)
Uh...... even though I suggested you have some more baby yu, you didn't have to go at it that hard......Yu?Yuyu?Marisa: It's Reimu's fault! Reimu must be a youkai that eats peni penis! Marisa was tricked by Reimu! Reimu that can't take it easy, go die easy!Or wait, I guess that's not what happened.Yugu? What happened to Marisa yu?MAWISHA'S MOST IMPORTANT PENIPENIIIIIIIII! CAN'T REFRESH ANYMOOOOOORE!