The role of cytomegalovirus in human disease is a still-evolving story. Hanshaw presented an excellent review article on the subject in 1981 in this publication; this current review is an update, with particular emphasis on new concepts in the epidemiology and prevention of cytomegaloviral infection and disease. Historically, evidence of infection with cytomegalovirus was first reported by pathologists in many parts of the world. They noted the presence of giant cells with intranuclear inclusions while examining a diversity of organs microscopically. Isolation of the virus and development of serologic techniques eventually enabled a definitive study of the agent, its pathogenesis and epidemiology. Biologically, it is one of the herpesviruses and, as such, is a DNA virus. Other members of the group include varicella-zoster, herpes simplex, and Epstein-Barr virus. Several different strains of cytomegalovirus exist, and they have specific characteristics which are of interest. The virus is cell associated and tends to be very labile; it has a tendency to become latent and may possibly have malignant potential. EPIDEMIOLOGY Prevalence Infection with cytomegalovirus is found throughout the world. Studies of prevalence in a number of diverse populations have indicated that cytomegaloviral infection is ubiquitous. The major differences in prevalence between populations are related to the speed of acquisition of infection in various geographic and socioeconomic settings.