Because of the sudden onset of "flu-like" symptoms in he vast majority of cases, followed by persistent illness and fatigue over several years, bolh RNA (retroviruses) and DNA (herpesviruses and enteroviruses) viruses have been suspected to be implicated in the pathogenesis of CFS. In recent years, evidence of the association of some viruses wilh CFS has progressed, whereas, with some others it has weakened considerably. Thus far, no single virus has been found to be the causative agent of CFS. Reactivation, however, of latent virus or viruses could contribute to the symptomatology of CFS by damaging the immune system either directly or indirectly. In this report we have provided a comprehensive review of the status of research on viral agents which have been investigated for their role in the pathogenesis of CFS.