Leading scientists of the day, including the world’s preeminent bacteriologist, Sir Graham Wilson and Harvard Virologist John Enders, who first isolated measles, warned against introducing a vaccine unless it provided lifelong immunity. Measles, they cautioned, would rebound with increased virulence and mortality as the vaccine forced the evolution of more virulent strains and shifted outbreaks away from children–biologically evolved to handle measles–to the elderly who could die from pneumonia, and young infants now unequipped with maternal immunity.