The mammalian immune system defends the organism against pathogens, and possibly cancer, but is known to become dysregulated with increasing age. This results in greater morbidity and mortality due to infectious disease in old people. The most important changes occur in T cell immunity, manifested sometimes dramatically as altered clonal expansions of cells of limited antigen specificity and a marked shrinkage of the T cell antigen receptor repertoire. At the same time, it was independently reported that CMV seropositivity was associated with many of the same T cell changes that were being identified as biomarkers of immune ageing. It has now become clear that CMV is commonly the driving force behind the oligoclonal expansions and altered phenotypes and functions of CD8 cells seen in most old people. These changes are much less obvious in centenarians and most extreme in people whom longitudinal studies have shown to possess an "immune risk profile". This is a cluster of immunological parameters of which CMV seropositivity is one component and which predicts incipient mortality in an elderly population. Taken together, these findings suggest the hypothesis that persistence of CMV as a chronic antigenic stressor is a major contributor to immunosenescence and associated mortality.