Cytomegalovirus (CMV) is a member of the Herpesviridae family, along with herpes simplex viruses 1 and 2, Epstein-Barr virus, and varicella-zoster virus. It is a double-stranded DNA virus with a protein coat and lipoprotein envelope. Similar to other herpesviruses, CMV is icosahedral and replicates in the host's nucleus. Replication in the host cell typically manifests pathologically with large intranuclear inclusion bodies and smaller cytoplasmic inclusions, and is accompanied by presence of CMV viral particles in the plasma.
Gross specimen of bowel showing ulceration secondary to cytomegalovirus colitis. 
Giant cell with inclusion body characteristic of cytomegalovirus colitis.
Between 50% and 80% of the world's population is seropositive for CMV. Initial CMV infection in the immunocompetent host typically is mild and goes undetected clinically. This is followed by a chronic latent state, during which the virus remains present within host cells, but viral proliferation is prevented by host cell-mediated immunity. Failure of immune containment may lead to reactivation with viral proliferation and severe systemic illness. Systemic CMV disease is characterized by fever, pancytopenia, and inflammatory changes in multiple organs including the liver and lungs, and in the retina. Colitis is a frequent manifestation of this acute systemic illness.
Patients are rendered susceptible to systemic CMV disease by treatment with immunosuppressive medications, or by illnesses that reduce cellular immunity, such as human immunodeficiency virus (HIV) infection. Acute systemic illness caused by CMV is particularly common following initial exposure in an immune compromised individual (in particular, in a CMV-negative transplant recipient who receives an organ from a CMV-positive donor).
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