There is a line of research into the "transmissible' phenomena of a lot of these neurodegen dz. Epidemiologically, it's probably not infectious from one person to another bc we don't see spouses getting PD or AD often. But there's interesting phenomena - i.e. one of the Parkinson's stem cell implantation trials got a lot of press after the trial (which failed, didn't show benefit) bc after subjects passed several years later and got autopsied, they found clumps of parkinsonian proteins (lewy bodies) on the histology slides of the implanted stem cells.
Similarly, there's some papers w/ mice w/ knockout Parkinsonian genes getting parkinsonian features and lewy bodies when injected w/ abnormal misfolded synuclein from another mouse.
What exactly to do w/ this, no one is entirely sure yet.