I was once in the hospital for a year, and I became very close friends with a gentlemen that had Bone Cancer. I watched him progress through the stages into hospice and eventually pass away. The pain he went through, and amount of bones broken or nerve pain symptoms, has lasted with me as a deep fear.

I would answer dementia, but I also made friends with those patients in the hospital too and tbh they were *mostly* happy.

Locked in syndrome. It's what happens after you have a massive stroke or cerebral hemorrhage yet survive. You're completely paralyzed, the only thing you can do is blink your eyes. Yet you're completely mentally/cognitively aware of what's going on you, you just can't respond to it at all.

Probably the most terrifying disease to have.

Fibrodysplasia Ossificans Progressiva (FOP). It's a rare genetic disorder where muscles, tendons, and connective tissues gradually turn into bone, progressively limiting movement and creating a "second skeleton" that traps the body.

I'm going to strongly say CRPS (complex regional pain syndrome). Many people can find some sort of remission or reduction of symptoms. But for those people who have long term CRPS are generally quite profoundly disabled.

It's a disease that just causes pain, immense pain without a reason. Like the highest pain levels recorded are from this, period. Like levels comparable to amputation without medication.

It develops typically after an injury or surgery but can also happen spontaneously or from something as small as a needle puncture. It often affects a limb. But it can spread to your whole body, jump to other limbs.

It can cause severe swelling and contracture to the point limbs become non functional.

Many people are so sensitive that a touch from the wrong fabric can put people in excruciating pain.

The daily pain these patients live with would put normal people in the hospital. For the most part people have no frame of reference for this type of pain and It never stops, ever, it's inescapable. Sometimes it just gets so bad people will do anything escape the pain including leaving this world.

There is no cure. If you manage to go into complete remission (not very common) it can come back at any time in your life, 10, 20 years later. It can be hard to find successful treatment options and a lot of people can find some relief or be stable at a "livable" but definitely disabled level. Fit example, my wife might need to take a solid dose of hydromorphone (the strong one) just to get to a manageable level of pain at a 7 or 8 out of 10 pain score. That is on top of a neurostimulator and a pain pump that delivers morphine directly into her spine 24/7. She will also getting another stimulator in her spine as it has spread to her other leg.

The worst or those who suffer from this, suffer an unimaginable existence for most people.

Honestly there's even more absolutely stuff with this but it's too long to cover.