Infectious mononucleosis, also called "mono," is a contagious disease. Epstein-Barr virus (EBV) is the most common cause of infectious mononucleosis, but other viruses can also cause this disease. It is common among teenagers and young adults, especially college students. At least 25% of teenagers and young adults who get infected with EBV will develop infectious mononucleosis. Enlarged spleen and a swollen liver are less common symptoms. For some people, their liver or spleen or both may remain enlarged even after their fatigue ends. Most people get better in 2 to 4 weeks; however, some people may feel fatigued for several more weeks. Occasionally, the symptoms of infectious mononucleosis can last for 6 months or longer.