65 My seriously-ill-but-feels-fine thing reminds me of some kind of mushroom that I've heard about. For about a month after consuming this mushroom (which Wikipedia is not helping me locate), you have no obvious symptoms, but meanwhile, all of your cells are taking irreversible damage to their chromosomes. And then you die.
After consuming this kind of mushroom, you'd experience some (potentially deadly) symptoms like those caused by other toxic mushrooms. Then (if you survive this long) you'd go into a "walking ghost" phase where (as mentioned) you feel (mostly?) fine, but your chromosomes are taking (or have already taken?) irreparable damage. And then symptoms worsen again, followed by death.
That's not what's happening to me, though. I had been experiencing some symptoms, but they were a bunch of different, random things that I didn't put together until shortly before you all heard about it.