I think I understood it a little better after rereading this chapter. The thing about the illness making you immortal is only a figure of speech, it just makes your body accept any kind of organ and blood transferral, so as long as you live in a hospital you're essentially immortal because doctors have it much easier to cure you. And the boy died of old age at 12 years old because as the teacher said, the illness stops the cells' replacement, so his body didn't make any new ones. The doctors dying of old age I think it's just a visual metaphor and the whole scene isn't supposed to be taken literally since they're discussing the meaning of the manga itself. The girl dying of old age... maybe the thing about her being killed and being able to revive was a lie to make the boy think he killed her (Doku could help faking her death), and she actually died of old age too? I don't think anything I've said makes full sense either but idk.