Introduction

Human cytomegalovirus (CMV) is a ubiquitous agent that rarely causes disease in healthy individuals but is an important pathogen in the immunocompromised or the immunologically immature foetus. Individuals at high risk are those with deficiencies in cell-mediated immunity and it is, therefore, believed that in the immunocompetent host the cell-mediated immune response is the most important form of defence against CMV (Grundy, 1991). In the immunocompromised, CMV infection can be associated with a wide range of symptoms. The most serious complication in allogeneic transplant recipients is CMV interstitial pneumonitis, which is associated with a high mortality (Meyers, Flournoy & Thomas, 1982), and the virus remains the most important infectious cause of death following bone marrow transplantation. In AIDS patients, CMV can cause sight-threatening retinitis (Collaborative DHPG Treatment Study Group, 1986) and serious disease throughout the gastrointestinal tract. Congenital CMV infection can be associated with disseminated disease, including in the CNS, and the virus is an important cause of deafness and mental retardation (Alford et al., 1990). Thus, CMV causes disease in many organ systems in various patient populations. However, even in the immunocompetent host, the virus is not eradicated from the body following primary infection and can persist in a latent form throughout life. Hence, whilst the normal host immune response is usually able to prevent disease associated with primary CMV infection, the virus ultimately evades the host response and becomes latent.