I am looking for a book that I read 25-30 years ago, but may be older. It is about how Parkinsons, MS, and cancer are contagious in the sense that they have the same viral cause, but manifests differently for each individual. Author may have been an M.D.

Perhaps ‘Plague Time : How Stealth Infections cause Cancers, Heart Disease, and other Deadly Ailments’ by Paul W Ewald. Published 2000.
Wow! I’m impressed with this service. I believe the book you mentioned IS the one I’ve been looking for and I thank you for responding so quickly!!