Epstein-Barr virus: A virus, abbreviated EBV, best known as the cause of infectious mononucleosis ("mono").

Infection with EBV is characterized by fatigue and general malaise. Infection with EBV is fairly common and is usually a transient and minor thing. However, in some individuals EBV can trigger chronic illness, including immune and lymphoproliferative syndromes. It is a particular danger to people with compromised immune systems, such as from AIDS.

EBV was at one time believed to be the cause of the chronic fatigue syndrome, but chronic infection with this virus actually produces a separate (if similar) disorder.

EBV is a double-stranded DNA virus in the herpes family of viruses. The virus is named for the English virologists Michael Anthony Epstein (1921-) and Yvonne M. Barr (1932-).