Virologic and pathogenetic aspects of cytomegalovirus infection

Cytomegalovirus (CMV) is a ubiquitous agent that rarely causes disease in immunocompetent humans but is an important cause of morbidity and mortality in the immunocompromised. A number of host and viral factors are associated with pathology following CMV infection. CMV can be found at many sites in the body but only causes disease at some of these and then only in certain patient populations. In some situations the mechanism underlying disease is direct viral replication, but in others, particularly CMV pneumonitis in allogeneic transplant recipients, an immunopathologic basis is strongly implicated. An important factor in the pathogenesis of infection and the expression of symptomatic disease is the source of CMV infection-whether it arises from an exogenous source or is due to reactivation of latent endogenous virus. Exogenous infection can occur in previously seronegative individuals or in those with prior exposure to the virus. In renal transplant recipients both types of exogenous infection have been associated with disease. Another factor that affects the interaction between CMV and its host is the modulating effect of the virus on the host immune response, the mechanism of which remains unknown.