From the open-access journal Immunity & Aging, a look at cytomegalovirus (CMV) and what it does to your immune system over the years: "Ageing is associated with multiple immune system dysfunctions. An important current direction for immunosenescence research is towards assessing the clinical impact of age-associated modifications of immunity and modulating them. ... Lifelong and chronic antigenic load may represent the major driving force for immunosenescence, which impacts on human lifespan by reducing the number of virgin antigen-non experienced T cells ... Gradually, the T cell population shifts to a lower ratio of naive cells to memory cells ... Thus, the repertoire of cells available to respond to antigenic challenge from previously unencountered pathogens shrinks. ... Many of the clonal expansions filling the individual's immune system seem to result from previous infections by persistent viruses, especially CMV, but also, to a lesser extent, EBV and possibly other herpesviruses. A high number of CD8+ cells are found to be specific for a single epitope of cytomegalovirus: in some individuals up to a quarter of circulating CD8 cells carry receptors for a single CMV epitope." CMV is cluttering your immune system with uselessly specialized cells, leaving you with too few capable defenders in later age - and thus shortening your life.