A herpes virus, hence the name. There is 6A and 6B.
Sex. Although data to support spread by heterosexual sex (PubMed) or blood transfusion is negative.
HHV-6A and HHV-6B infects almost everyone by age 2.
Exanthem subitum (also known as roseola infantum or sixth disease. Usually 6B.
HHV-6A is more neurovirulent, and is more frequently found in diseases such as multiple sclerosis and t may have a role in gliomas (PubMed).
HHV-6B reactivation occurs in transplant recipients, with encephalitis, bone marrow suppression and pneumonitis.