The Anelloviridae Family: Ubiquitous Commensals, Immune Biomarkers, and Unresolved Pathogenic Potential

The family Anelloviridae comprises small, non-enveloped, single-stranded circular DNA viruses known for their global ubiquity and extreme genetic diversity. Initially discovered as Torque Teno Virus (TTV) in a patient with post-transfusion hepatitis of unknown origin, anelloviruses are now recognized as near-universal components of the human virome and are widespread among animals. They possess a small (~2–4 kb) negative-sense genome, exhibit high mutation and recombination rates, and establish persistent, typically asymptomatic infections. In healthy individuals, anelloviruses are acquired in infancy and maintain a lifelong, dynamic presence characterized by individual-specific genotypic diversity. Despite their prevalence, their biological role remains unclear. Current evidence suggests anelloviruses are non-pathogenic commensals that exist in equilibrium with the host immune system. This unique relationship has positioned them as promising non-invasive biomarkers for monitoring immune function. Clinically, particularly in transplantation contexts, anellovirus DNAemia has emerged as a surrogate marker of immune competence, with viral loads inversely correlating with immune activity. Conversely, elevated anellovirus levels have been associated with various diseases, including respiratory, hepatic, neurological, and neoplastic conditions, though no causal link has been definitively established. It is widely hypothesized that anelloviruses may function as opportunistic indicators of immune dysregulation or inflammation, rather than primary pathogens. This review examines the taxonomy, genome organization, host range, and host-virus dynamics of Anelloviridae, emphasizing their dual identity as both virome residents and potential immunological markers, while highlighting unresolved questions regarding their role in human disease.