What is Infectious Mononucleosis (Mono)?

A young couple kisses.

What is Infectious Mononucleosis ("Mono")?

Infectious mononucleosis is a common infection caused by the Epstein-Barr virus (EBV). It is also known as glandular fever, the kissing disease, and mono. Symptoms of infectious mononucleosis include fatigue, sore throat, fever, and swollen lymph nodes. There is no treatment for mono, but supportive care involves resting and drinking plenty of fluids. The illness resolves without treatment, but symptoms may last from several weeks to months.

Professor Sir Anthony Epstein

What is the Epstein-Barr virus?

The vast majority of cases of infectious mononucleosis, more than 90%, are caused by the Epstein-Barr virus, which is in the herpesvirus family. The virus was discovered by professor Sir Anthony Epstein and Yvonne M. Barr in England. The virus was named "mononucleosis" because it's associated with an increase in certain white blood cells called lymphocytes in the bloodstream.

This electron microscopic image of two Epstein-Barr virus virions (viral particles) shows round capsids -- protein-encased genetic material -- loosely surrounded by the membrane envelope.

What is the Cause of Mono?

It's estimated up to 95% of adults in the U.S. have evidence of Epstein-Barr virus antibodies in their blood by the time they are 35 to 49 years old. The presence of antibodies means a person has been infected in the past. EBV is the most common cause of infectious mono although there are other illnesses that produce similar symptoms.

A group of students socialize and hang out.

What are the Risk Factors for Mono?

Anyone can suffer from Epstein-Barr virus infection. It's most common in those between the ages of 5 and 25; however, 90% to 95% of adults demonstrate evidence of past infection. About 1% to 3% of college students are infected with mono every year. EBV can be transmitted through saliva (kissing) and via blood and sexual contact.

A young couple kisses, and a boy sneezes, spraying germ droplets.

How is Mono Spread?

Mono is easily spread through casual contact. Among teens, kissing is the most common means of transmitting mono since the virus is present in saliva. Epstein-Barr virus is airborne and can be transmitted by droplets released when an infected person coughs or sneezes. Sharing food or beverages with an infected person can also spread EBV infection.

A young girl, contagious with mono, wears a mask.

How Long is Mono Contagious?

Many people come into contact with the Epstein-Barr virus and never develop an active infection. Instead, these people develop antibodies to the virus, which gives them immunity. The incubation period for mono is 4 to 6 weeks, meaning it takes that long after exposure for someone to develop symptoms. Mono is contagious for a few weeks or longer during an active infection.

A girl suffering from mono lies in bed.

What Are the Symptoms of Mono?

Symptoms of mono are different in the beginning of the illness compared to later in the course of the illness. Initial symptoms within the first three days may include chills, loss of appetite, and feelings of lack of energy or malaise. More intense symptoms may occur after the first three days and can include profound fatigue, swollen lymph nodes in the neck, fever, and a severe sore throat. Throat symptoms can be so intense it leads people to seek medical attention.

A person diagnosed with mono shows the signs of a reddened sore throat and whitish coating on the tonsils.

What Are the Signs of Mono?

The common signs of mono include swollen, red tonsils, enlarged lymph nodes in the neck, and a fever that ranges from 102°F to 104°F. About one-third of people who have mono have a whitish coating on their tonsils. Approximately 50% of people with mono have swollen spleens. The spleen is located in the upper left abdomen. Liver enlargement may occur. A small number of people with mono, about 5%, develop a body-wide red, splotchy rash. The rash looks similar to that which occurs with measles.

A doctor examines a patient's throat for signs of mono.

How is Mono Diagnosed?

A variety of tests may be used to diagnose mono. In the early stages, a doctor may suspect mono based on a patient's signs and symptoms. A health care professional may order tests to rule out other potential causes of the symptoms, such as strep throat. Numbers of white blood cells called lymphocytes may be elevated in the early stages of mono. Lymphocytes may also look different when examined under a microscope. Blood tests such as the heterophile antibody test and the monospot test can be used to confirm mono diagnosis. These tests measure levels of antibodies in the blood. Antibodies are typically detectable after two to three weeks into the illness. Another potential tell-tale sign of mono infection is abnormal liver function tests.

A woman sleeps as she recovers from mono.

What is the Usual Course and Treatment of Mono?

Mono, like many viral illnesses, just needs to run its course and it usually resolves without treatment. Antiviral drugs are not effective for treating mono. There's some evidence these medications prolong mono infection. Treatment for mono is supportive and focuses on symptom relief. Aches, pains, fever, and headache can be treated with acetaminophen. Plenty of rest, sleep, and fluids can help ease symptoms. Painful, sore throat -- one of the most uncomfortable symptoms of mono -- is typically the worst during the first 5 to 7 days of the infection. Sore throat resolves and gets much better in the subsequent 7 to 10 days.

A young male is exhausted and still feeling the effects of infectious mononucleosis.

Are There Any Long-term Effects of Mono?

Some people are fatigued or tired for months after Epstein-Barr virus infection. It's best to avoid contact sports for 6 to 8 weeks after the appearance of symptoms as a blow to the abdomen could damage or burst an enlarged spleen. EBV may be present in the saliva for up to 18 months after infection. A person who has symptoms for more than 6 months is said to have chronic EBV infection.

A patient is upset after the doctor's diagnosis.

What Are the Complications of Mono?

Mild inflammation of the liver (hepatitis) is often a complication of mono. Hepatitis associated with mono is not usually serious and does not require treatment. Potential rupture of the spleen is a possible dangerous complication of Epstein-Barr virus infection. Severe complications of mono are rare and almost never cause death in healthy people. Rare and severe potential complications of mono include inflammation of the heart (myocarditis), inflammation of the heart lining (pericarditis), inflammation of the brain (encephalitis), and destruction of red blood cells (hemolytic anemia). People who have compromised immune systems, including those with AIDS or those who take medications that depress immune function, are more likely to have aggressive cases of mono. EBV has been linked to cancers including lymphoma and one type of Hodgkin's disease.

A doctor consults with a patient.

Infectious Mononucleosis at a Glance

Here are key facts about infectious mononucleosis:

  • Epstein-Barr virus (EBV) is a contagious infection that causes infectious mononucleosis.
  • The illness is spread by casual contact, usually via saliva. Mono has an incubation period of 4 to 6 weeks.
  • The vast majority of adults have antibodies against EBV, meaning they have been infected with the virus and are immune to mono.
  • Fatigue, severe sore throat, swollen lymph nodes, and fever are common symptoms of mono.
  • Specialized blood tests are used to confirm the diagnosis of mono.
  • Mono can be associated with an enlarged spleen and liver inflammation (hepatitis).
  • One should avoid contact sports during active mono illness and during recovery because of the possibility that the spleen can rupture.



  1. iStockphoto / Designs of Integrity
  2. Copyright University of Bristol
  3. Public Library of Science - Citation: (2005) Virus Proteins Prevent Cell Suicide Long Enough to Establish Latent Infection. PLoS Biol 3(12): e430
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  5. BigStockPhoto / Yuri Arcurs
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  8. Photo Courtesy of Allie Dilbeck
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  13. BigStockPhoto / Marcin Balcerzak


  • American Academy of Family Physicians: "Mononucleosis."
  • Odumade, O. A., et al. "Progress and Problems in Understanding and Managing Primary Epstein-Barr Virus Infections." Clinical Microbiology Review 24.1 (2011): 193-209.
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