Cytomegalovirus (CMV) is a member of the Herpes family. A primary infection of CMV to healthy young people is not significant (results in Mononucleosis). In immunocompromised patients, CMV is latent and recurs. It is also implicated in organ transplant rejections.
Epstein-Barr virus (EBV) + CMV together activate polyclonal cells, which are modulated against totally irrelevant antigens, causing a mechanism that produces organ rejection. CMV pneumocytis in bone-marrow transplant patients is a very serious and often life-threatening disease.