Epstein-Barr virus (EBV) is the etiological agent for heterophile positive infectious mononucleosis (IM). It is also an oncogenic herpes virus associated with African Burkitt's lymphoma (BL),nasopharyngeal carcinoma (NPC) and AIDS-associated B-cell lymphomas. EBV is also among a group of viruses that have been associated with chronic fatigue syndrome (CFS), although the etiology of CFS still remains unknown.Findings may lead to hypothesize that EBV enzymes, such as the dUTPase, have the capacity to induce immune dysregulation of the T-cell and NK cell responses and that this immune dysregulation produces immunopathology that results in the symptoms that we call CFS.