<p>Research on host-associated microorganisms has been dominated by barrier-surface communities, especially the gut microbiome. Yet, humans also harbor a less visible layer of long-term microbial association: persistent intracellular extraintestinal microbes (PIEM) that reside in internal tissues or circulating cells outside classical mucosal and barrier niches. These organisms—including latent viruses and other persistent intracellular or cell-associated microbes—are typically studied as pathogens, but their biological impact may extend beyond overt infection. We propose that PIEMs represent an underappreciated regulatory dimension of the human metaorganism. Their importance lies not in biomass, but in persistence, strategic cellular tropism, latency–reactivation dynamics, and the capacity to tune host setpoints—that is, durable baseline thresholds of immune, vascular, metabolic, and tissue responses. Human cytomegalovirus (HCMV) provides a tractable model for this framework because of its high prevalence, lifelong latency, broad cell tropism, and ability to deploy regulatory proteins and noncoding RNAs, including circulating viral miRNAs. Here we argue that HCMV illustrates a continuum from regulated persistence to disease-promoting dysregulation, with implications for inflammatory, vascular, neoplastic, and age-related disorders. At the same time, many disease associations remain correlational, and distinguishing microbial drivers from passengers or biomarkers will require rigorous causal frameworks. Expanding microbiome thinking beyond the gut may therefore open a new frontier in systems medicine, linking persistent internal host-associated microbes to human homeostasis, disease susceptibility, and precision diagnostics.</p>