Cytomegalovirus infection (CMV) is a viral infection that rarely causes obvious illness. The virus that causes CMV is part of the herpes virus family and, like other herpes viruses, may become dormant for a period of time and then be reactivated. CMV affects young children mainly, but it is estimated that by age 30 in the United States, half of all adults are, or have been, infected. The virus can pass from an infected, pregnant mother to her child through the shared blood supply (umbilical cord). Physicians recognize three clinical forms of CMV.