When I saw this, my first thought was this tragic examination [1] of ~half a dozen cases of people developing Alzheimer's like symptoms with a similar lack of history - and the underlying thing they had in common was that they had all had a treatment done years prior which sourced cadaver tissue in a way that did not preclude other things being in the treatment bundle, which was eventually banned over fears of something like CJD being transmitted, which they assume was the underlying cause.
[1] - https://www.nature.com/articles/s41591-023-02768-9