From Webmd:
an acute infectious disease that causes changes in the leukocytes; it is caused by the Epstein-Barr virus, and is usually transmitted by direct oral contact (which is why it is sometimes called the "kissing disease"). It occurs more frequently in the spring and affects primarily children and young adults. Although epidemics have been reported, some authorities doubt that the disorder has been the same in all instances. Called also glandular fever.
SYMPTOMS. Generally, after an incubation period of one week to several weeks, headache, sore throat, mental and physical fatigue, severe weakness, and symptoms typical of influenza develop. Skin rashes may also occur.
Diagnosis can be confirmed by the finding of a marked increase in the number of mononuclear leukocytes present in the patient's blood. These mononuclear cells belong to the lymphocyte class. Often there is an increase not only in lymphocytes but also in atypical lymphocytes. Another diagnostic test that indicates mononucleosis is the Paul-Bunnell heterophil agglutination test, which demonstrates the presence of certain antibodies capable of causing clumping of cells in a sample of sheep's blood.
In about 8 to 10 per cent of all cases, the liver is involved and jaundice occurs, resulting in a condition that resembles infectious hepatitis. In rare cases, the heart, lungs, and central nervous system may also be affected. The spleen may become enlarged; one of the complications, serious but rare, is rupture of the spleen. The lymph nodes and spleen may both remain enlarged for sometime after other symptoms have disappeared.
TREATMENT. Treatment is chiefly symptomatic. Rest is especially important in the early stages of the disease, or later if the liver is involved. There is as yet no specific treatment for mononucleosis, and no immunization is available. Headache and sore throat may be relieved by aspirin and gargles.
Although the more obvious symptoms may disappear after a period of rest, sufficient rest and curtailed activities must be maintained in order to improve the patient's severely weakened condition and prevent recurrence. There is often mental as well as physical fatigue, especially among students, and in these cases some mental depression may accompany convalescence.
Chronic, lingering infectious mononucleosis occurs in some patients. They experience profound fatigue, low-grade fever, swollen lymph glands, a sore throat, and aching muscles and joints. These symptoms can persist for months or years, and often cause the patient to be labeled neurotic because of a lack of objective evidence of disease. The condition can be definitively diagnosed by blood testing for antibodies to the Epstein-Barr virus. Unfortunately, diagnosis can provide psychological relief only, as there is no effective treatment or cure for the disorder.