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Primary infection with human herpesviruses 6 and 7 (HHV-6 and HHV-7) during early childhood causes permanent latent infection, usually without any ill effects; only a small percentage of primary infections will lead to exanthem subitum. Like other herpesviruses. HHV-6 and HHV-7 can be reactivated at any time if host defence mechanisms become defective (e.g. in transplant recipients, AIDS, tumour patients). HHV-6 can be reactivated under such conditions and cause a variety of clinical problems, such as exanthems along with interstitial pneumonia or hepatitis for example. In addition, the reactivated virus may influence the course of autoimmune and proliferative diseases such as systemic lupus erythematosus and Hodgkin's disease. While, HHV-7 may be associated with similar disorder, more systematic studies are needed to clarify the clinical implications and the pathogeetic mechanisms of both viruses.

Human herpesvirus-6 is a lymphotropic virus which infects susceptible individuals during the first year of life and usually causes life-long latency. In a variable percentage primary infections are followed by a short acute disease, exanthema subitum. Older individuals may suffer from infectious mononucleosis-like illnesses or from Kikuchi-Fujimoto's disease. In addition, there is a fairly wide spectrum of lymphoid and hematopoietic diseases or autoimmune disorders, which are associated with elevated titers of HHV-6 antibody, and from which replicating virus may be isolated. Such diseases include atypical polyclonal lymphoproliferation, Hodgkin's disease, chronic fatigue syndrome and systemic lupus erythematosus. The present article reviews the current knowledge of such associations.