Despite progress in understanding the pathophysiology of human cytomegalovirus (HCMV) infections, its manifestations in the immune compromised host are frequently associated with high morbidity and mortality. In this setting, HCMV disease can develop e.g. following immune suppression as a result of reactivation of latent HCMV acquired earlier in life. The mechanisms leading to establishment of latent infections and their subsequent reactivation are not clear. It is also unknown whether HCMV exists in a latent form with limited viral gene expression or as a persistent infection with normal virus transcription.