Cytomegalovirus (CMV) is a virus belonging to the herpes family. The disease occurs worldwide and is transmitted by human contact. About four out of five people over age 35 have been infected with cytomegalovirus, usually during childhood or early adulthood. In most of these people, the disease is so mild that it's overlooked. However, CMV infection during pregnancy can be hazardous to the fetus, possibly leading to stillbirth, brain damage, other birth defects, or neonatal illness. Cytomegalovirus has been found in the saliva, urine, semen, breast milk, feces, blood, and vaginal and cervical secretions of infected persons. Transmission usually takes place through contact with these infected secretions, which harbor the virus for months or even years. Immunodeficient patients (AIDS patients or who have those who have received transplanted organs), are at high risk of contracting CMV infection. Recipients of blood transfusions from donors with positive CMV antibodies are at some risk.

Cytomegalovirus is probably spread through the body in lymphocytes or mononuclear cells to the lungs, liver, and central nervous system where it often produces inflammatory reactions. In most patients, the disease usually runs a benign, self-limiting course. However, immunodeficient patients and those receiving immunosuppressives may develop pneumonia or other secondary infections.