Infectious mononucleosis, commonly called "mono," is a disease caused by the Epstein-Barr virus (EBV). EBV is one of the most common viruses worldwide. Most people are exposed to EBV as children, but the infection often goes unnoticed because they experience minimal symptoms. The majority of people have been exposed to EBV by the age of 35 and have developed immunity. However, when EBV infection occurs during late adolescence or early adulthood, there is a 35% to 50% chance of developing mono.