Cytomegalovirus, also often labeled CMV, is a type of Herpes virus also known (in humans) as human herpesvirus type 5, or HHV-5. It is in the betaherpesvirinae subfamily of Herpes. The name means "cell very big virus".
CMV especially attacks salivary glands and may also be devastating or even fatal to fetuses. CMV viruses are found in many mammal species, but generally are specific only to that species.