Essentially, Autochthon is the only Primordial that has the ability to die.
All of the others? Death is not and was never a part of them until the Exalted forced it on them[1]. They were immortal and perfect, save for Autochthon who was frail and can die. His sickness, his cancer is the touch of the Void, of Oblivion, and the echoes of the Neverborn Engine of Extinction that is yet to be. As he weakens, the Engine grows stronger, and if Autochthon dies, the Engine will become fully realized.
And unfortunately, Auto's sickness is woven throughout every fiber of his being, infecting his very essence and Charms. According to the Devs, even if undergoes Fetich Death(which normally, essentially cause a Primordial to become something completely different from what they were) it will just make things worse.
His cancer manifests as Gremlin Syndrome and Voidtech, both of which are evil as fuck and freak the Yozi right the fuck out.
[1]Afterwords, some of the Yozi dabble in death and Necromancy, notably the Ebon Dragon and Oramus, Dragon Beyond The World, but most of them stay far away from it.