Cytomegalovirus (CMV) is a member of the herpes virus family, which also includes herpes simplex virus types 1 and 2, varicella-zoster virus, and Epstein-Barr virus. CMV is a common viral infection that, in the absence of HIV or other immunocompromising conditions, remains latent and is not associated with serious illness. In immunocompromised people, however, CMV may be a major cause of disease because the suppressed immune system may permit reactivation of the virus. More than ninety percent of people with acquired immunodeficiency syndrome (AIDS) show evidence of prior CMV infection and may continue to harbour inactive or latent virus.