Chronic fatigue syndrome (CFS) is an illness characterized by disabling and long-lasting fatigue and associated with several somatic symptoms such as myalgia, arthralgia, headache, recurrent sore throat, and neurocognitive dysfunction. Frequently, the onset of the illness is preceded by flu-like symptoms, which indicates that immune abnormalities or a viral infection might be involved in the pathogenesis. Although no specific agent has been definitively linked to CFS, several different viruses e.g., herpesviruses, enteroviruses, and retroviruses, have been investigated.
You can read the full article here: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC86845/
Source: Enbom M, Linde A, Evengård B. No evidence of active infection with human herpesvirus 6 (HHV-6) or HHV-8 in chronic fatigue syndrome. J Clin Microbiol. 2000 Jun;38(6):2457. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC86845/