Cytomegalovirus (CMV) (from the Greek cyto-, "cell", and megalo-, "large") is a genus of viruses in the order Herpesvirales, in the family (150-200nm in diameter [hiv virus 120nm]) Herpesviridae, in the subfamily Betaherpesvirinae. Human herpesvirus 6 (HHV-6) is the common collective name for human herpesvirus 6A (HHV-6A) and human herpesvirus 6B (HHV-6B). These closely related viruses are two of the nine herpesviruses known to have humans as their primary host.