Mononucleosis is a viral infection causing fever, sore throat, and swollen lymph glands, especially in the neck. 
Mono will leave you weak and feeling tired for weeks or maybe even months.
Mono goes away on its own, but lots of rest and good self-care can help you feel better.
It is caused by the Epstein-Barr Virus (EBV). EBV is a member of the herpes virus family. 
The designation "mononucleosis" refers to an increase in one type of white blood cells (lymphocytes) in the bloodstream relative to the other blood components as a result of the EBV infection.