Indeed - it could equally be explained by dementia/Alzheimer’s being a contagious disease, such as a prion or similar protein malformation, which quite a bit of current research seems to suggest, and it simply had not become endemic at that point.
It disappoints me that something which severely impacts the lives of perhaps one third of humans isn't better understood.
If it were a prison disease, surely with ~2 billion data points of those with/without dementia, we would have isolated exactly what molecule causes it and how it gets transmitted from person to person.
