This is a DNA virus in the herpes group, and 30% of children get roseola (fever and rash); but 80% of adults are seropositive..so, there must be many other forms of disease and/or assymptomatic acquisition of immunity. Seropositivity is defined as presence of IgG antibody (Ab). The initial seroconversion by a primary infection and/or reactivation or re-infection is implied by the additional presence of IgM antibody. Immunodeficient patients got the virus in 25-40% of cases. The HHV-6 IgG antibody level boosts with infections of other HHV types, especially CMV. [warning]

• in less than 20% of adults...never infected; non-immune status
• those previously exposed but either non-immune or have sub-detectible levels of immune (IgG) Ab.

Infants or Young Children

• Roseola:  (synonyms: roseola infantum, exanthema subitum,  sixth disease) High fever 3-5 days followed by hours to days neck and trunk rash
• Otitis
• Gastroenteritis

Adults

• Chronic fatigue syndrome: some cases (not sure)  associated with HHV-6 viremia (PCR detection of HHV-6 DNA extracted from WBCs  in these patients)
• Mononucleosis-like syndrome
• Hepatitis syndrome: from mild LFT elevation to fulminant hepatitis
• Pulmonary syndrome: mild to severe pneunmonitis
• broncho-alveolar-cell carcinoma of lung 
• cross-reactive rise of IgG with other infections, especially CMV, mean that a rising IgG titer (alone) is insufficient for HHV-6 diagnosis

Other names for this exact or approximate agent are:   

• HHV-6, HBLV, human B-lymphotrophic virus

References:

• Advance for Admin. of the Laboratory (magazine), page 78, March 2000.