What is mononucleosis?

Mononucleosis (mono) is a viral infection that is usually caused by the Epstein-Barr virus (EBV). This virus spreads through infected saliva. In children, this may happen through coughing or by handling or chewing toys that have been contaminated by the virus. In teens, the virus can spread through sharing food and drinks or toothbrushes or through close, personal contact such as kissing.

Other viruses that can cause a mononucleosis-like illness include cytomegalovirus, Toxoplasma gondii, adenovirus and viral hepatitis.

Signs and symptoms of mono

A child or teen can have infectious mononucleosis from EBV for some time before showing any symptoms. This is known as the incubation time of a virus. The incubation time for mono can be up to 30 to 50 days in teens, but it is shorter for children.


In children, mono typically shows very few or no symptoms. Any minor symptoms are usually limited to:

  • a cold
  • a mild to moderate fever, which is not dangerous and usually lasts less than two weeks.

Teens and young adults

Symptoms for this age group start slowly over one or two weeks. They can include:

  • a sore throat
  • tiredness, loss of energy and body aches
  • large red tonsils covered with pus
  • swollen lymph nodes in the neck, armpits, groin and elsewhere
  • a fever for 10 to 14 days
  • headache
  • nausea and loss of appetite
  • abdominal pain
  • a rash
  • a slightly enlarged spleen
  • a slightly enlarged liver
  • jaundice.

Most teens have only mild symptoms for a week. Even those with severe symptoms often feel completely better after two to four weeks.

How a doctor diagnoses mono

Your child’s doctor will diagnose mono by examining your child and asking about any symptoms. They may also order blood tests to help confirm the diagnosis or rule out more serious (but less common) causes of your child's symptoms.

Complications of mono

Although it is not common, mono can sometimes cause more serious complications. These include:

  • dehydration
  • an enlarged spleen
  • breathing problems
  • a rash.


Dehydration occurs when a sore throat prevents a child from drinking enough fluids. Try to prevent dehydration by giving your child pain medicines for their throat and small, frequent amounts of fluids.

Enlarged spleen

Mono can cause your child's spleen to become enlarged (get bigger) and leave it at risk for injury. A hit to the stomach could rupture the enlarged spleen and cause internal bleeding. This is an emergency.

You can keep your child’s spleen safe by having your child avoid contact sports (see below), preventing constipation and avoiding heavy lifting.

If your child has sudden, severe pain in the abdomen (belly), call 911 or take them to the emergency department right away.

Breathing problems

Mono may cause breathing difficulties if your child's throat is swollen and their airway is partially blocked. This could happen if there are enlarged tonsils, adenoids or other lymph tissue at the back of the throat.

A child may describe a feeling of "something being stuck" if they have a sore throat or difficulty breathing. Your child's doctor can examine their throat to see if there is a risk of blockage and may prescribe medicines to reduce any swelling.


Sometimes patients with mono develop a rash after taking certain antibiotics. This rash is copper-coloured and itchy and can develop after taking ampicillin or, less frequently, amoxicillin. If your child develops this kind of rash, ask their doctor about stopping the antibiotic. Your child may need a different antibiotic instead or may not need an antibiotic at all, as antibiotics will not treat mono.

What your child's doctor can do for mono

Mononucleosis is a viral infection. Antibiotics cannot treat a virus, so, in most cases, your child's doctor will suggest medicines to ease fever or pain. But if your child’s tonsils are so large that they are almost touching, the doctor may prescribe steroid medication to reduce the swelling.

Taking care of your child with mono at home

Fever and pain medicines

Give your child acetaminophen or ibuprofen to ease the discomfort of swollen lymph nodes or fever. Always follow the instructions on the package.

Relief for sore throat

If your child finds it painful to eat and drink, try:

  • giving soft foods that are easy to swallow, such as soups, ice cream, pudding or yoghurt
  • avoiding highly salted, spicy or acidic (vinegar or citrus fruits) foods if they make the pain worse
  • letting your child take their fluids through a straw or sippy cup
  • having your child gargle with warm salt water (older children and teens)
  • giving your child ice cubes and lozenges to suck (older children and teens). Do not give lozenges or cubes to young children, as they are a choking hazard.


Make sure your child drinks plenty of fluids. Your child is getting enough fluid if:

  • they urinate (pee) at least two or three times a day
  • they have moist eyes and tears when they cry
  • their mouth is moist and has saliva.

Rest and activity

Your child will likely need to rest, but they do not need to be isolated or stay in bed. Usually children slow down when they have a fever and then become more active when they feel better.

Returning to school

Children can go back to school when their fever is gone and they can swallow normally. Most children are ready to return to their normal routine in two to four weeks.


All children with mono should avoid contact sports for at least four weeks or until cleared by their doctor. This is to prevent damage to the spleen if it became enlarged. 

How to prevent mono from spreading

Mono is most contagious while your child has a fever. After the fever is gone, the virus is still carried in the saliva in small amounts for up to six months. To prevent the spread of the mono virus, your child should use separate drinking glasses and utensils. Teens with mono should avoid kissing until the fever has been gone for several days.

When to see a doctor

Call your child’s regular doctor if:

  • your child cannot drink enough fluids and is not peeing
  • your child is not back to school two weeks after being diagnosed with mono
  • your child still has symptoms after four weeks
  • you have other questions or concerns, for example about your child's return to sports.

Call 911 or go to the nearest emergency department if your child:

  • is dehydrated
  • has problems breathing
  • has abdominal pain, especially high on their left side.

Key points

  • In children, mono is a type of viral infection that can cause no symptoms or a mild febrile illness.
  • Teens and young adults with mono often have other symptoms with their fever, including sore throat, headaches and swollen lymph nodes.
  • Keep your child comfortable and hydrated while their body fights the illness.
  • Avoid sports and heavy lifting. Injury to the spleen can cause severe internal bleeding.

Shawna Silver, MD, FRCPC, FAAP, PEng