The newly isolated virus, human herpesvirus 6 (HHV-6), causes exanthem subitum (or roseola infantum) as the primary infection. Liver dysfunction and neurological disorders are observed during exanthem subitum. Seroepidemiological studies have shown that almost all children are infected with HHV-6 about 6-24 months after birth. This virus also causes other diseases including lymphadenitis and infectious mononucleosis-like disease. HHV-6 has cellular tropism for CD4+ lymphocytes, in which it replicates in vivo. Its transmission is probably airborne early in life, mainly from mother to child, because it is often secreted in the saliva of healthy adults having antibody to HHV-6. Latent infection follows the primary infection and the virus can often be reactivated under conditions of immunosuppression such as AIDS or organ transplantations. The site of its latent infection is not yet known, but could be monocytes/macrophages.