The Epstein–Barr virus (EBV), also called human herpesvirus 4 (HHV-4), is one of eight known human herpesvirus types in the herpes family, and is one of the most common viruses in humans.

It is best known as the cause of infectious mononucleosis (glandular fever). It is also associated with particular forms of cancer, such as Hodgkin's lymphoma, Burkitt's lymphoma, gastric cancer, nasopharyngeal carcinoma, and conditions associated with human immunodeficiency virus (HIV), such as hairy leukoplakia and central nervous system lymphomas. There is evidence that infection with EBV is associated with a higher risk of certain autoimmune diseases, especially dermatomyositis, systemic lupus erythematosus, rheumatoid arthritis, Sjögren's syndrome, and multiple sclerosis. Some 200,000 cancer cases per year are thought to be attributable to EBV.

Infection with EBV occurs by the oral transfer of saliva and genital secretions.

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