Eating a bit of inactivated poliovirus as a child generates a population of memory immune cells that can inactivate poliovirus that you are exposed to as an adult, even if there has been no exposure in the intervening years. What keeps these cells alive? Ku et al. show that these memory cells are sustained by a delicate balance among several cytokines. Interleukin 15 (IL-15) and IL-7 keep the cells dividing slowly; IL-2 acts as a brake on this process to keep cell division under control.