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. The mono test will reveal whether you have this infection.