What Is Mononucleosis?

Mononucleosis (mono) is a viral infection that causes a sore throat and fever. Cases often happen in teens and young adults. It goes away on its own after a few weeks of rest.

What Causes Mono?

Mononucleosis (mah-no-noo-klee-OH-sus), or infectious mononucleosis, usually is caused by the Epstein-Barr virus (EBV). Most kids are exposed to EBV at some point while growing up. Infants and young kids infected with EBV usually have very mild symptoms or none at all. But infected teens and young adults often develop the symptoms that define mono.

Other viruses, such as cytomegalovirus, can sometimes cause a mono-like illness too.

What Are the Signs & Symptoms of Mononucleosis?

Signs of mono usually show up about 1–2 months after someone is infected with the virus. Its most common symptoms are sometimes mistaken for strep throat or the flu. These include:

  • fever
  • sore throat with swollen tonsils that may have white patches
  • swollen lymph nodes (glands) in the neck
  • being very tired

A person also can have:

  • headaches
  • sore muscles
  • weakness
  • belly pain with a larger-than-normal liver or spleen (an organ in the upper left part of the belly)
  • skin rash
  • loss of appetite

Is Mono Contagious?

Mono is contagious. It spreads from person to person through contact with saliva (spit). It's nicknamed "the kissing disease" because it can spread through kissing. It also spreads through coughing and sneezing, or when people share something with spit on it (like a straw, drinking glass, eating utensil, or toothbrush).

Mono can also spread through sexual intercourse and blood transfusions, but this is much less common.

People who've been infected carry the virus for life, even after symptoms stop and even if they had no symptoms. The virus is then "dormant," or inactive. Sometimes the dormant virus "wakes up" and finds its way into a person's saliva. This means that they can be contagious from time to time, even when they have no symptoms. 

How Is Mono Diagnosed?

To diagnose mono, doctors do an exam to check for things like swollen tonsils and an enlarged liver or spleen, common signs of the infection. Sometimes the doctor will do a blood test.

How Is Mono Treated?

The best treatment for mono is plenty of rest and fluids, especially early in the illness when symptoms are most severe. There is no specific medicine for mono, but acetaminophen or ibuprofen can help to relieve fever and aching muscles. Antibiotics will not help, because they're only effective against bacteria. Someone with mono who takes an antibiotic often ends up getting a rash.

Never give aspirin to a child who has a viral illness because its use has been linked to Reye syndrome, which may cause liver failure or even be fatal.

How Long Does Mono Last?

Mono symptoms usually go away within 2 to 4 weeks. In some teens, though, the tiredness and weakness can last for months.

Can Mono Be Prevented?

There is no vaccine to protect against the Epstein-Barr virus. But you can help protect your kids from mono by making sure that they avoid close contact with anyone who has it.

Many people who have mono won't have symptoms, but they can still pass it to others. So kids should wash their hands well and often, and not share drinks or eating utensils with others, even people who seem healthy.

What Else Should I Know?

Mono can make the spleen swell for a few weeks or longer. An enlarged spleen can rupture, causing pain and bleeding inside the belly, and needs emergency surgery. So doctors recommend that kids who have mono avoid contact sports for at least a month after symptoms are gone. Your child should stay away from rough play, heavy lifting, and any strenuous activities until the doctor says it's OK.

In most cases, mono symptoms go away in a matter of weeks with plenty of rest and fluids. If they seem to linger or get worse, or if you have any other questions, call your doctor.

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Note: All information is for educational purposes only. For specific medical advice, diagnoses, and treatment, consult your doctor. © 1995-2021 KidsHealth®. All rights reserved. Images provided by The Nemours Foundation, iStock, Getty Images, Veer, Shutterstock, and Clipart.com.