Mononucleosis is an illness caused by infection with a virus.
Mono is an illness mainly of adolescents and young adults. In developed nations, the majority of the population has been exposed to the Epstein-Barr virus by the age 18. Students most often experience a painful sore throat with tonsillar enlargement. Less often you can experience loss of appetite, nausea, vomiting, diarrhea or measles-like rash. Diagnosis is based on a patient's symptoms and physical exam.