I just read Madness and Memory - The Discovery of Prions - A New Biological Principle of Disease by Nobel Laureate Stanley Prusiner, M.D., published 2014. He gives convincing evidence that PD, AD, ALS, all the neurodegenerative diseases are prion based. A 2011 thread on this site: "Breakthrough! Alpha-synuclein responsible for progression" begins the discussion.

Prions are proteins with no DNA or RNA, not viruses. Simplistically put, they can mis-fold and go rogue, taking over normal proteins and wiping us out. He ends the book with a couple of sobering points: "Once so mysterious, degenerative brain diseases are beginning to reveal the mechanisms by which they compromise nervous system function." and "For the wars on cancer and heart disease there are a myriad of advocates and pipelines filled with hundreds of promising drugs--for neurodegenerative diseases, the ranks and the pipelines are virtually empty."