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Human herpesvirus type 6 (HHV-6), a newly recognized human herpesvirus first described in 1986, is morphologically similar to other herpesviruses but is distinguishable from all of them by some unique in vitro biological effects, specific antigenic analysis, and patterns of endonuclease restriction digests of DNA. In vitro HHV-6 exhibits tropism mainly for T lymphocytes, but it also infects other cells, including B lymphocytes, monocytes-macrophages, glial cells, and fibroblasts. Because HHV-6 causes frequent infection in infants and children, a seroprevalence rate of antibody to this virus of up to 80% has been reported in the United States. Infection in infancy develops as levels of maternal antibody wane, thus resulting in either subclinical infection or an acute febrile illness termed exanthema subitum. Primary infection acquired later in life causes a disease resembling acute infectious mononucleosis. Since HHV-6 shares the capacity to establish latent infection with other herpesviruses, frequent viral reactivation is probably the explanation for the high incidence of serologically proven active HHV-6 infection found simultaneously with active infection due to other herpesviruses as well as in the presence of various immune deficiency conditions. [ABSTRACT FROM PUBLISHER]