Epstein-Barr virus: A virus in the herpes family that is best known as the cause of infectious mononucleosis (also called mono and glandular fever). Abbreviated EBV. EBV infection is characterized by fatigue and general malaise. Infection with EBV is common and is normally temporary and minor. 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, including those with AIDS. Treatment is with antiviral medication and rest. Also known as human herpesvirus 4 (HHV-4).