Human herpesvirus 6 (HHV-6) is a T-lymphotropic herpesvirus, which infects almost all children by the age of two years and persists lifelong. Two distinct variants of HHV-6, HHV-6A and HHV-6B, have been described, and the latter has been shown to be a common cause of acute febrile illnesses in young children, including exanthem subitum (roseola). HHV-6 has also been associated with a number of neurological disorders, including encephalitis and seizures, and the virus has been postulated to play a role in acquired immunodeficiency syndrome (AIDS), multiple sclerosis (MS) and chronic fatigue immunodeficiency syndrome (CFIDS). This review provides a critical summary of research conducted on HHV-6.

Human herpesvirus 7 (HHV-7) is a recently described T-lymphotropic herpesvirus, which infects almost all children by the age of three years and persists lifelong, with the shedding of infectious virus in saliva. HHV-7 is similar to human herpesvirus 6 (HHV-6) in its genetic content and in many of its biological properties, which include the ability to cause at least some cases of exanthem subitum (roseola). Despite these similarities, important differences between HHV-7 and HHV-6 exist, including the fact that HHV-7 binds to the cellular CD4 molecule and uses this protein as a necessary component of its receptor, while HHV-6 binds to a different (and unknown) receptor. Furthermore, the pathogenesis and sequelae of HHV-7 infection remain very poorly understood. This review provides a critical summary of research on HHV-7.