Infectious mononucleosis is an infectious, very widespread viral disease caused by the Epstein- Barr virus( EBV), a type of Herpes virus. It is most common among adolescents and young adults and is characterized by fever, sore throat and fatigue, along with several other possible signs and symptoms. Primary infections with EBV in childhood either remain silent or are accompanied merely by mild signs and symptoms in the throat and respiratory tract, whereas the highest rate of infectious disease occurs in older children and young adults, especially college students where the transmission of virus takes place through saliva during unprotected sexual acts. EBV infects the epithelium of the oropharynx and salivary glands. Potential mortal complications include splenic rupture, bacterial superinfections, hepatic failure and the development of viral myocarditis.Meningitis, encephalitis, hemiplegia and transverse myelitis may take place.Diagnostic criterion is the presence of 50% lymphocytes with at least 10% atypical lymphocytes (large, irregular nuclei), while the person also has fever, pharyngitis and adenopathy. Diagnosis is to be confirmed by a serological test. The antiviral drug valacyclovir has recently been shown to lower or eliminate the presence of the EBV in subjects afflicted with acute mononucleosis.