Definition of Mononucleosis

Mononucleosis: Infection with the Epstein-Barr virus
(EBV, human herpesvirus 4, HHV-4) in which there is an increase of
white blood cells that have a single nucleus (monocytes). The
infection can be spread by saliva. Its incubation period is four to
eight weeks. Symptoms include fever, fatigue, sore throat, and
swollen lymph glands. Mononucleosis can cause liver inflammation
(hepatitis) and spleen enlargement; vigorous contact sports should be
avoided to prevent spleen rupture. It is less severe in young
children.

Most people exposed to EBV do not develop mononucleosis:
most adults carry an antibody against EBV in their blood, meaning
they have been infected with EBV at some time. Treatment includes
rest, pain medication, and in some cases anti-viral medication.