This is a hard thing to write, but I hope that this story at least provides a little insight into what was a very difficult journey for my father.
My father died in February of 2007, after a long battle with mysterious symptoms which included a decline in his physical stamina as well as progressive dementia.
A little background: My father did some of his post-doctoral work at a cyclotron, researching elementary particles and nuclear physics. At that time, the risks of radiation exposure were less obvious than they are today, and there is little doubt that he was exposed to unhealthy amounts of radiation. Many of the research facilities of that era are now to "hot" (radioactive) to safely enter.
One of the earliest symptoms he faced was a marked decline in his physical stamina. After much testing, the best the doctors could come up with was a diagnosis of "myelodysplasia." This is a broad term that basically means dysfunctional blood. In his case, his blood seemed to have a reduced ability to deliver oxygen, as well as a reduced ability to fight infection.
It is suspected that radiation exposure can cause myelodysplasia, possibly by damaging the ability of bone marrow to produce healthy blood cells.
By all accounts, my father was a very intelligent man. One of the things that intelligent people do is to develop ways to deal with their own shortcomings. Ironically, these tactics certainly delayed the diagnosis of dementia, to the point where things were pretty far along before any alarms went off. By the time it became clear that he needed help with managing his conditions, it was really too late to do anything.
As best as anyone could tell, the myelodysplasia may have caused, or at least intensified his mental decline.
One of the silver linings typical of Alzheimer's type dementia is that the sufferer is often blissfully unaware of their decline. While this may seem like a terrible thing to say, I can tell you that my dad was definitely aware of his decline, and it was a terrible thing for him. He had always enjoyed his intellect, so his mental decline, and his awareness of that decline made the ordeal doubly cruel.
I leave treatments as a footnote, since every medicine the doctors tried just seemed to intensify his confusion, sometimes triggering dramatic hallucinations.