Epstein-Barr virus (EBV), herpesvirus that is the major cause of infectious mononucleosis and is associated with a number of cancers, particularly lymphomas in immunosuppressed persons, including persons with AIDS. Epstein-Barr is a ubiquitous virus, so common that it has been difficult to determine whether it is the cause of certain diseases or whether it is simply there as an artifact. In Third World nations, most children are infected with EBV; in most industrialized nations, about 50% of the people are infected. Research has found that all of the lymphomas associated with AIDS and most lymphomas in other immunocompromised persons are connected with latent EBV infection. EBV has been found in biopsy tissue of patients with Hodgkin's disease, breast cancer, and some smooth muscle tumors. EBV also was formerly suspected as the cause of chronic fatigue syndrome (originally named chronic EBV syndrome).