Human herpesvirus 6: A herpes virus that apparently lies dormant in many people, and is most likely to cause problems when the immune system is compromised by disease, as in AIDS patients, or by deliberate immune suppression, as in organ transplant patients. Abbreviated HHV-6. There are two forms of HHV-6: A and B. A is rare and is acquired in adulthood. B is relatively common, is usually acquired in childhood, and is associated with roseola. Both HHV-6 A and B can reactivate at a later date and are believed to contribute to diseases of the bone marrow and/or central nervous system in some patients, including fatal encephalitis, chronic fatigue syndrome, and possibly multiple sclerosis. Diagnosis is made via rapid blood culture or other blood test. Treatment is experimental, but antiviral drugs or beta interferon may be tried.