Mononucleosis Also known as mono and kissing disease, it is an acute infection that affects the respiratory, liver and lymphatic systems. It spreads by close contact. It is common in adolescents and young adults. It is rare in people older than 35. It is usually harmless and goes away on its own. It is rarely serious and causes extreme fatigue. The most serious complication is an enlarged spleen. May explode on direct contact, such as when playing contact sports.