Infectious Mononucleosis


What Is It?

Infectious mononucleosis, commonly called mononucleosis or “mono,” is an illness caused by a viral infection, most often the Epstein-Barr virus. Mononucleosis has been nicknamed the “kissing disease” because Epstein-Barr virus commonly is transmitted in saliva during kissing. However, sneezes and coughs also can transmit the virus occasionally. In about 15 percent of cases, mononucleosis is caused by cytomegalovirus and human herpes virus-6, viruses that, like Epstein-Barr virus, are members of the herpes family. HIV infection, hepatitis viruses and even the parasitic infection toxoplasmosis can mimic infectious mononucleosis.

Mononucleosis typically occurs the first time a person is infected with Epstein-Barr virus. Once a person is infected, the virus remains alive in the body for the rest of his or her life. After the initial infection, it rarely produces any kind of illness, but it can be transmitted to others. The initial infection with Epstein-Barr virus does not always cause mononucleosis. It often causes only a mild illness, like a cold, or no illness at all. Epstein-Barr virus permanently infects more than 90 percent of the people on earth, but it causes mononucleosis only in a minority of them. In developed nations, such as the United States, mononucleosis most often develops between the ages of 15 and 25, although it can occur at any age.


The first symptoms of mononucleosis typically include fever, headaches, muscle aches and unusual fatigue, such as the need for 12 to 16 hours of sleep daily. These symptoms are followed very shortly by:

  • Sore throat
  • Enlarged lymph nodes
  • Chills
  • Joint aches
  • Loss of appetite and slight weight loss
  • Nausea and vomiting, occasionally
  • A red rash, usually on the chest — much more common if the person has recently taken the antibiotics ampicillin or amoxicillin (both sold under several brand names)
  • Abdominal pain
  • Enlarged spleen

Other rare symptoms include jaundice (yellow skin and eyes), difficulty breathing, coughing or irregular heart rhythms. In rare cases, an enlarged spleen can rupture. This is most likely to occur if the person is struck in the abdomen, possibly during contact sports or other activities. The spleen is a small organ near the stomach that produces infection-fighting white blood cells, and purges worn-out red blood cells from the circulation. Untreated, a ruptured spleen can cause life-threatening internal bleeding.


Your doctor will ask you about your medical history and current symptoms, as well as about your recent exposure to anyone with mononucleosis or mono-like symptoms.

During a physical exam, your doctor will look for signs of mononucleosis, especially fever, a reddened throat with enlarged tonsils (possibly covered with pus), swollen lymph nodes in the neck and elsewhere, an enlarged spleen (located in the upper left side of the abdomen) and a red rash, usually on the chest.

Your doctor also will perform blood tests to help make the diagnosis. The results of these blood tests may not become abnormal until the person has been ill for a week.

Expected Duration

Symptoms of mononucleosis usually are most intense during the first two to four weeks of the illness. However, mononucleosis symptoms, especially fatigue, can sometimes last for several months.


This disease is most contagious during its acute stage, while the affected person still has a fever.

Someone with mononucleosis does not need to be kept isolated from others. However, to help prevent the spread of mononucleosis, the patient should avoid kissing temporarily. Some authorities recommend that a person with mononucleosis also should avoid sharing food, drinks or eating utensils with other people during the first few weeks of the illness, although the value of this is unproven.


Mononucleosis usually is a self-limited illness, and most treatment focuses on making the person more comfortable. Because there is currently no medical cure for mononucleosis, the basic treatment plan usually calls for getting plenty of rest and fluids and addressing any uncomfortable symptoms. For example, cold drinks, frozen desserts and gargling with salt water can help to relieve minor sore throat pain. Ibuprofen (Advil, Motrin and others) or acetaminophen (Tylenol) can be taken to fight fever and body aches.

Steroids, such as prednisone, are not recommended, but may be prescribed if the tonsils are so swollen that it is hard to breathe, or when certain rare complications occur, such as anemia. To protect the spleen from rupture, a person with mononucleosis should avoid strenuous activities, especially contact sports, for at least four weeks.

When To Call A Professional

Call your doctor if you develop the symptoms of mononucleosis. If you have been diagnosed with mononucleosis, contact your doctor immediately if your breathing becomes difficult or noisy, if you experience intense pain in the upper left side of your abdomen, or if your symptoms seem to be getting worse instead of better after one to two weeks.


Most patients with mononucleosis recover completely. About 10 percent of people with mononucleosis develop strep throat (a bacterial infection) that needs to be treated with antibiotics.

Johns Hopkins patient information

Last revised:

Diseases and Conditions Center

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All ArmMed Media material is provided for information only and is neither advice nor a substitute for proper medical care. Consult a qualified healthcare professional who understands your particular history for individual concerns.