HHV.6 (or HBLV) herpesvirus was isolated in 1986. This virus has a specific tropism for T cells. It is ubiquitous. Most subjects are infected in early childhood. HHV.6 is responsible for exanthema subitum. Its possible involvement in the aggravation of immunodeficiency syndromes and its action on malignant haematologic disorders, has been suggested but has not been clearly demonstrated.