Mononucleosis (or "mono") is an infectious disease caused by the Epstein-Barr virus (EBV). EBV is a member of the Herpes Virus family.
The infectologist is a medical specialist and diseases caused by infectious agents such as bacteria, viruses, fungi and parasites.
Remember that for the proper selection of health professional that treats mononucleosis you must consider the nature, severity and symptoms you are presenting.