Human herpesvirus 6 discovered in 1986 is the most ancient human herpesvirus shown by molecular characteristics. Variant B infects children under the age of 2 years by droplets from asymptomatic virus shedding adults occasionally causing exanthema subitum. The virus infects CD4+ macrophages and lymphocytes; subsequently establishes lifelong latency and persistence with occasional shedding through the saliva. This variant frequently reactivates in bone marrow and organ transplant recipients with concomitant immunosuppression causing even fatal complications. It is a cofactor in the pathogenesis of multiple sclerosis, chronic fatigue syndrome, Hodgkin and non-Hodgkin lymphomas. The direct consequences of variant A infection and latency in CD4+ cells are not known. It transactivates HIV infection in vitro and in humans, and facilitates tumor progression induced by human papilloma viruses. Pathogenic effects of both variants are mediated by altered cytokine and chemokine profiles. Serological differentiation of the two variants is unreliable; however, it is possible by using PCR. Ganciclovir, foscarnet and cidofovir can be used for treatment and chemoprophylaxis of severe complications.