A perplexing question in immunology has been, how do immune cells remember an infection or a vaccination so that they can spring into action decades later? Research led by scientists at the University of California, Berkeley, in collaboration with investigators at Emory University, has found an answer: A small pool of the same immune cells that responded to the original invasion remain alive for years, developing unique features that keep them primed and waiting for the same microbe to re-invade the body.