In 1965, Jack was picked to hand over a busload of children to the 456. He thought he was chosen because of his immortality, but actually, it was because people figured that he wouldn't care. He did care, of course, and vividly remembers trading in twelve children in the 456's "protection racket". If he hadn't done it, the Earth would have been devastated by a plague, engineered by the aliens. Jack had already lived through the Spanish Flu -- he couldn't bear to see the Earth suffer like that again.