become the local authority on a rash of early-onset alzheimers cases in and around medellin. >> francisco loperawere getting disease very early in the life. >> stahl: it all began many years earlier, back in the 1980s, when lopera was a young medical resident. he had read about small numbers of people scattered around the world who had developed alzheimers in their 40s. so when a 47-year-old man came into his medellin clinic with alzheimer's-like symptoms, he was intrigued, and decided to investigate. you met this one man, and you decided to go to where he was from? >> lopera: i decided to go to the town where he was living. >> stahl: lopera learned that the man's father and grandfather had also lost their memories in their 40s. then, a few years later, another similar patient came into the clinic, this time a 42-year-old woman from a town 40 miles away. dr. lopera's then-nurse, lucia madrigal, asked if any of her relatives also started losing their memories when they were young. >> lucia madrigal( translated ): they told us yes, that the father, the uncles, the grandfather, the great grandfathe