In the 1960s, epidemiologists studying the long-term prognosis of survivors of the 1918 Spanish Influenza began to notice an unusual trend. Those who were born between 1888 and 1924 – meaning they were either infants or in young adulthood at the time of the pandemic – appeared to have been two or three times more likely to develop Parkinson’s disease at some point in their life than those born at different times.