When the experts, of which my mother had been one, say a disease is insidious they mean the symptoms may not be obvious at first. Other definitions:
An insidious disease is any disease that comes on slowly and does not have obvious symptoms at first. The person is not aware of it developing.
In my book: a disease that lets you fake it for a rather long time.
Insidious in common parlance has a very strongly negative sense: evil that lurks and only gradually emerges. That part of the definition seems just right for thinking about dementia. Dementia is an evil disease, more so than the fact that all diseases, all illnesses are evil.
Is it worse to lose your mind or your body? Let's set aside for a moment that one's mind is part of one's body. This dichotomy may or may not be particularly useful for understanding health and illness. Yet, if you talk to the elderly (and I don't mean functional bluehairs and greybeards, I mean the folks staring hard at the end of their life), they make a very clear distinction in this.
My father lived well into his 90s. He was physically slower, but could still get roused for a good argument about what my (my!) CV should look like, why this or that politician was full of it, or whatever cause-du-jour attracted his attention. But he could barely move; years of playing handball, running and lifting weights had destroyed his knees. I am pretty sure he was depressed for all sorts of reasons, including that my mother's dementia precluded their massive arguments about everything. Yet, he could set that depression aside to tell me the truth about whatever he had just read on the internet. His favorite site, btw, was Arts & Letters Daily (a good compendium of ideas, thoughts and stuff you had no idea was as interesting as it proves to be).
Physical decline is less insidious: its there and obvious and in your face each day when you wake up in pain. From his perspective, his body, gradually to be sure, failed to keep its half of the bargain with his mind. When my father died, he was tired. He went to sleep and didn't wake up. My mother never thought she'd outlive him. But she did. By many years. But she never knew this, and never will.
She was in complete and entire denial that anything was wrong, and that is part of the cruelty. My mother came from less-than-working-class family. My grandparents were illiterate, and had gone to work in factories as young children in New York City. Her mind, her brain, her personality is what carried her out of poverty to being a med school professor in the days when such women were counted in single digits at any school. I can imagine there was nothing more terrifying to her than losing that by which she defined herself. Her response differed only slightly from aging athletes who know, deep down know, that they cannot recreate the triumphs of their youth.
This is the evil part, but what about insidious? All of my immediate family had large arguments about what was happening to my mother. I saw signs before my sibs and father. One of them actually said: "you really hate Ma, don't you? You want her to be demented." My mother and I never got along. She was a marvelous mentor, and I loved her, as a daughter loves a good mother. But, we fought bitterly through my adolescence. We struggled to define a good relationship in my adulthood. But irrespective of that relationship, I was not anxious to see her ill or see her demented. We fought, but I did not, do not, wish dementia on anyone. I didn't know why this scared me so much at the time, but the caretaking I've done over the last 15 years gives me exquisite hindsight.
My mother has died. What was it, what, 3 weeks ago, already? More? It still feels like yesterday to me. I know it was time for her. But, for me, why did it have to be so soon?