Mononucleosis is caused by the Epstein-Barr virus and is therefore sometimes called that however "mono" is the nickname for infectious mononucleosis which is considered to be an official name.
Official name of the mononucleosis virus is Epstein Barr Virus, commonly known as EBV. It's a virus transmitted via body fluids, and most commonly it is transmitted through saliva. It's also known as "illness of kissing".
