Also called CMV. The majority of the adult population has this virus, which is a strain of herpetoviridae (herpes), although not considered a STD.

From CancerWeb:

Infected cells enlarge and have a characteristic inclusion body (composed of virus particles) in the nucleus. Causes disease only in utero (leading to abortion or stillbirth or to various congenital defects), although can be opportunistic in the immunocompromised host.
The American Red Cross gives preferential treatment to donors who are CMV-negative, since this blood is given to infants, children, and others with weak or compromised immune systems. They even stamp your donor card with "CMV NEG" next to the blood type. As the lady at the Red Cross told me, CMV can produce minor symptoms in adults, rarely amounting to much. However, in children and infants, and people with compromised immune systems, it can be very damaging, even lethal.