Cytomegalovirus (CMV) (from the Greek cyto-, "cell", and -megalo-, "large") is a viral genus of the Herpesviruses group: in humans it is commonly known as HCMV or Human Herpesvirus 5 (HHV-5). CMV belongs to the Betaherpesvirinae subfamily of Herpesviridae, which also includes Roseolovirus. Other herpesviruses fall into the subfamilies of Alphaherpesvirinae (including HSV 1 and 2 and varicella) or Gammaherpesvirinae (including Epstein-Barr virus). All herpesviruses share a characteristic ability to remain latent within the body over long periods.

HCMV infections are frequently associated with salivary glands, though they may be found throughout the body. HCMV infection can also be life threatening for patients who are immunocompromised (e.g. patients with HIV, organ transplant recipients, or neonates). Other CMV viruses are found in several mammal species, but species isolated from animals differ from HCMV in terms of genomic structure, and have not been reported to cause human disease.

HCMV is found throughout all geographic locations and socioeconomic groups, and infects between 50% and 80% of adults in the United States as indicated by the presence of antibodies in much of the general population. Seroprevalence is age-dependent: 58.9% of individuals aged 6 and over are infected with CMV while 90.8% of individuals aged 80 and over are positive for HCMV. HCMV is also the virus most frequently transmitted to a developing child before birth. HCMV infection is more widespread in developing countries and in communties with lower socioeconomic status and causes the most significant viral cause of birth defects in industrialized countries.