Symptoms of Mononucleosis

Mononucleosis (mono) is a viral infection (typically caused by the Epstein-Barr virus) that most often affects teens and young adults. Typical symptoms in those age groups are a sore throat, fever, swollen neck glands, enlarged tonsils, and fatigue. In younger children, there may be only mild symptoms or none at all. Older adults are more likely to have jaundice and may not have a sore throat or swollen glands. It's common for people to have different combinations of symptoms that range from mild to severe.

mononucleosis symptoms
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Frequent Symptoms

The incubation period for mono is about four to six weeks, which means that you or your child might not develop symptoms until a month after being exposed to someone else with the infection.

For most children and teens, mononucleosis gets off to a slow start with typically three to five days of mild symptoms such as a headache, malaise, and fatigue.

The classic symptoms of mono then follow:

  • A severe sore throat
  • Red, swollen tonsils covered in pus
  • Swollen lymph glands (lymphadenopathy) in the neck and armpits, but also possible in the groin
  • Fever between 100 and 103 degrees that may last one to two weeks, and often peaks in the afternoon or early evening
  • Continued malaise and fatigue, which can be extreme
  • Achy muscles and headache
  • A rash that is pink and measles-like, more frequently seen after taking antibiotics
  • Nausea
  • Abdominal pain

The combination of symptoms, how severe they are, and how long they last vary from person to person. The younger the child, the less severe and the shorter the illness.

An enlarged spleen (splenomegaly) or liver can develop after two or three weeks of illness. Spleen enlargement is seen in about half of the cases at some point in the illness. Jaundice, which causes a yellow tinge to the skin and the whites of the eyes, is a sign of liver involvement.

Young children may have only brief, mild symptoms that last just a few days. In teens and young adults, mono is notorious for lasting a long time, although the usual course is two to four weeks in that age group. Some symptoms, such as fatigue, can last several more weeks to six months.

Rare Symptoms

Less common symptoms of mononucleosis include chest pain, cough, shortness of breath, high heart rate, hives, neck stiffness, nosebleed, and sensitivity to light. Swollen or inflamed testicles can also develop. 

Young Children

Younger children usually have much more mild symptoms of mononucleosis as compared to teens and young adults. An infant might have no symptoms or mild symptoms. Small children with mono may just be a little irritable and have a decreased appetite. On the other hand, they may also have mild upper respiratory infection symptoms, such as a cough, runny nose, or mild fever.

Some kids have atypical or unusual cases of mono—for instance, they develop an enlarged gland but no sore throat or fever. Or the only symptoms they have are a sore throat or a fever. In either case, one clue these symptoms may be due to mono is that they linger for much longer than seems normal.

Mono Often Goes Undetected

You may never know a child has mono unless they have blood tests done showing they have the atypical lymphocytosis or antibodies that indicate mono. Later in life, a positive test for the Epstein-Barr virus (EBV) may then reveal the past infection. The good news is that no treatment is usually given for mononucleosis other than relieving symptoms, so there was no missed opportunity for medical intervention.

Older Adults (40 and Older)

Older adults often show a fever lasting longer than two weeks and may not show the other common symptoms of swollen glands and sore throat. Their symptoms can persist longer. They are more likely to have jaundice and lab tests will show a high bilirubin level and liver enzyme levels and their white blood cell count will not be as elevated or show as many lymphocytes as is seen in younger people.


Complications of mono can develop and can be quite serious. They include the following.

  • Anemia: Mono can result in your red blood cells dying and being removed from your bloodstream by the spleen sooner than normal. Your bone marrow might not be able to keep up with this more rapid turnover, causing anemia.
  • Hepatitis with jaundice: Liver involvement is usually mild, but you should not drink alcohol while you have mono symptoms because of this.
  • Ruptured spleenThe spleen may become enlarged due to mono and, though rare, can subsequently rupture—an emergency that can cause massive, often life-threatening internal bleeding. It is important that you avoid contact sports and strenuous activities when you have mono, as they can cause this complication. The spleen is not required for good health, but you may be more at risk of infection by Streptococcus pneumoniae and some other germs if it is removed.
  • Nervous system complications: Though rare, the nervous system may also be affected by mono. Related complications include Guillain-Barre syndrome, Miller Fisher syndrome, seizures, meningitis, and Bell's palsy.
  • Fulminant EBV infection: It is rare, but people with weakened immune systems due to HIV, immunosuppressive therapy for an organ transplant, or X-linked lymphoproliferative disorders can get uncontrolled EBV infection and die from mononucleosis.
  • Heart inflammation: The most common (although still rare) heart complication with mono is inflammation of the sac around the heart, which is known as pericarditis. This can lead to atrial fibrillation. The virus can also infect the heart muscle and cause myocarditis.

There are also specific complications to be aware of for particular groups of individuals:


Airway obstruction from enlarged tonsils is possible in young children and it can require hospitalization. Your pediatrician might mistake the symptoms of mononucleosis for a bacterial infection (such as strep throat) and prescribe an antibiotic, such as ampicillin, amoxicillin, or related penicillin-like antibiotics. These antibiotics won't work because mono is a viral infection. Moreover, children sometimes develop a bad rash as a result of these medications.

Pregnant and Breastfeeding Women

Infectious mononucleosis caused by Epstein-Barr virus appears to have little to no effect on pregnancy outcomes, although there is some possible correlation with early delivery and lower birth weight.

Some women have reactivation of EBV during pregnancy. It is possible for Epstein-Barr to be passed to a newborn during birth. However, babies often have no symptoms when they have an EBV infection, so this is not a health concern. Breast milk can contain the virus, but it isn't clear whether this can produce an infection in a child.

When to See a Healthcare Provider

You should see the healthcare provider when you have the symptoms of mononucleosis so you can get a diagnosis and rule out other diseases that may have different treatment recommendations. Don't rely on self-diagnosis.

Besides EBV, other viruses can bring about mono-like symptoms. These include cytomegalovirus (CMV), adenovirus, human immunodeficiency virus (HIV), rubella, hepatitis A, and human herpesvirus-6. The parasite Toxoplasma gondii also can cause mono-like symptoms. If you are pregnant or may become pregnant, some of these other illnesses can be a risk for you or your baby. You may be given more tests to identify the cause of your mono symptoms.

Mononucleosis Doctor Discussion Guide

Get our printable guide for your next healthcare provider's appointment to help you ask the right questions.

Doctor Discussion Guide Man

After being diagnosed you may contract other illnesses that need medical attention. You can get strep throat on top of mono, for example. If you or your child has a severe sore throat, one that is lingering, or swollen tonsils that are making it hard to breathe or swallow, see your healthcare provider. Strep throat can be diagnosed with a rapid strep test. Antibiotics are needed to cure strep throat and avoid complications. You should also see the healthcare provider if there are any breathing difficulties due to enlarged tonsils.

Signs of an Emergency

The signs of a ruptured spleen include a sudden, sharp abdominal pain on the upper left side. You need to get to a hospital immediately and it's appropriate to call 9-1-1. A ruptured spleen usually requires blood transfusion and may require splenectomy surgery to remove the spleen and stop the internal bleeding.

The symptoms of mono usually get better after four to six weeks. If they are continuing, you should see your healthcare provider as you may, in fact, be dealing with a different issue. Your healthcare provider may perform additional tests to identify the cause of your symptoms.

Frequently Asked Questions

  • What are the stages of mononucleosis?

    Mono typically progresses through three distinct phases:

    • Incubation: You're infected with the Epstein-Barr virus (EBV) that causes mono but do not yet have symptoms.
    • Acute: You have active symptoms.
    • Convalescent: Acute symptoms have subsided, but you still feel tired and weak.
  • What does the beginning of mono feel like?

    When you're first infected with the Epstein-Barre virus, you aren't likely to feel anything different, as it can take several weeks for symptoms to appear. When they do, you may feel like you have a typical viral infection until you develop a severe sore throat and your tonsils are red and speckled with white pus.

  • Does EBV always cause mono?

    No. Children who become infected with EBV rarely develop symptoms. Of young adults between 18 and 22, 75% have classic mono symptoms, 15% have symptoms not typically associated with the disease, such as abdominal pain, nausea, and vomiting, and 10% are asymptomatic.

  • Can mono get better without treatment?

    Eventually, but it takes time. Mononucleosis is a viral infection, so there are no medications to treat it—it has to run its course. You can help speed your recovery along by getting plenty of rest and drinking lots of fluids.

14 Sources
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Additional Reading

By Vincent Iannelli, MD
 Vincent Iannelli, MD, is a board-certified pediatrician and fellow of the American Academy of Pediatrics. Dr. Iannelli has cared for children for more than 20 years.