mononucleosis, infectious, acute infectious disease of older children and young adults, occurring sporadically or in epidemic form, also known as mono, glandular fever, and kissing disease. The causative organism is a herpesvirus known as Epstein-Barr virus. The disease occurs most often in patients between the ages of 15 and 35. The virus is present in the saliva; it is usually spread by sharing a glass or kissing. Symptoms usually take 30 to 50 days to develop.

Diagnosis of mononucleosis follows the exhibition of a large number of abnormal white blood cells (lymphocytes) on microscopic blood examination. These blood cells have a single nucleus that give the disease its name. Symptoms are varied but include enlarged lymph nodes, sore throat, fever, enlarged spleen in about half the cases, and excessive fatigue. Occasional rashes and throat and mouth infections occur. Liver inflammation is common. Fatalities are very rare and, when they do occur, usually result from splenic rupture. General therapeutic measures include bed rest and treatment of symptoms.

or glandular fever

Common infection, caused by Epstein-Barr virus. It occurs most often at ages 10–35. Infected young children usually have little or no illness but become immune. Popularly called “the kissing disease,” it is spread mostly by oral contact with exchange of saliva. It usually lasts 7–14 days. The most common symptoms are malaise, sore throat, fever, and lymph-node enlargement. Liver involvement is usual but rarely severe. The spleen often enlarges and in rare cases ruptures fatally. Less frequent features include rash, pneumonia, encephalitis (sometimes fatal), meningitis, and peripheral neuritis. Relapse and second attacks are rare. Diagnosis may require blood analysis. There is no specific therapy.

Learn more about mononucleosis, infectious with a free trial on Britannica.com.