Detailed serologic screening showed an antibody prevalence to HBLV (HHV-6) in the general population of 26% if very strict criteria for antibody positivity were applied. Lower and borderline antibody titers yet may be found in up to 63% of the population. Only 17% of these persons have clinical symptoms; in the majority infection remains silent. HHV-6 infection apparently occurs already quite early in life, and initial symptoms can occur, such as short-term high fever, sore throat, local lymphadenopathy and skin rash. Lesions disappear without specific treatment. The frequency of positive antibody tests at higher titers rises in patients with immune deficiency and with atypical lymphoproliferative diseases to 60 and 75%. The rise in antibody titers is associated in patients with immune deficiency by characteristic shifts of blood lymphocyte populations, essentially by increase in immature T-lymphocytes. Highest titers are found in patients with lymphoproliferative syndromes, yet the percentage of atypical lymphoid cells harboring the viral genome is low (about 2% of seropositive patients). Thus it appears, that HBLV, similar to other herpesviruses such as Epstein-Barr virus, usually causes a silent seroconversion, yet may be associated with variable clinical pathology when persisting in an active state. Its pathogenic effect might be rather a cofactor contributing to immune disturbance than overt oncogenicity.