Aging of the global population is a critical public health issue. Immunosenescence is a core mechanism of aging. The herpes virus, cytomegalovirus (CMV), can establish lifelong latency in bone marrow stem cells after its entry into a human host. Cytomegalovirus latency is ubiquitous in aging populations and cytomegalovirus seropositivity is close to 100% in elderly populations. In addition, latent CMV in bone marrow stem cells can chronically induce immune responses through differentiation-dependent reactivation. The process of reactivation causes expansion of a large proportion of CMV- specific T cell clones with reserve na?ve T cells being gradually consumed. Thereafter, the diversity of adaptive T cells is reduced and inflammatory cytokines are consistently expressed at low levels, both of which are part of the core mechanism of immunosenescence. However, it is difficult to trace CMV latency and low level reactivation in bone marrow stem cells. The development of single cell-based RNA sequencing technology, however, enables the mechanism of latent CMV activation in bone marrow stem cells to be investigated in detail.