If you’re one of the five million people affected by Alzheimer’s disease, you know firsthand the toll it takes on families.  It's a disease that strips people of their memories and their identity and as yet, there’s no proven way to prevent or cure it.  But that could change as a result of groundbreaking research being done here in Boston and across the country. WGBH News Reporter Sean Corcoran starts the story in San Diego.