Human memory T cell pools proliferate and differentiate at varying rates that are determined by the frequency of lifelong antigenic re-encounter with different specific antigens. An important question concerning immunity is whether certain specific pools of memory T cells are driven to exhaustion in elderly subjects, a pertinent point in view of increasing human life expectancy. An emerging consensus is that cytomegalovirus (CMV), a β-herpesvirus with a prevalence of 60–90% worldwide, is an agent that induces specific T cells to extreme differentiation. The question that begs to be answered is whether this can explain why CMV seropositivity and the presence of highly differentiated CMV-specific T cells are included in a cluster of immune parameters that have been shown recently to predict the early mortality of elderly humans.