Human herpesvirus 6 (HHV-6) is found in peripheral blood leukocytes and is considered the agent of roseola. The normal prevalence of HHV-6 antibody is high. Nearly 100% of the population will demonstrate antibody at midlife with titers declining with increasing age. Immunofluorescence assays are used for detection of antibody recognizing HHV-6. Crossreactions with other herpes viruses, i.e., Cytomegalovirus and Herpes Simplex have not been found. In order to serologically demonstrate primary infection, it is necessary to detect a significant change in titer.