Mononucleosis Fact Sheet

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Infectious mononucleosis ("mono") is caused by a virus

The Epstein-Barr virus (EBV) that causes mononucleosis is a type of herpes virus. The virus infects a certain type of white blood cell called “B lymphocytes.”

The virus is shed in saliva and spreads through direct contact

EBV spreads through contact with the saliva of another person who has the virus. Kissing and contact with hands or toys soiled with infected saliva are common ways to spread the virus. EBV is shed in the saliva during the illness and for many months after infection. The virus can reactivate later and be shed again from the mouth and throat.

Symptoms to look for may include:

  • Fever
  • Sore throat
  • Swollen glands (especially in the back of the neck)
  • Tiredness

Symptoms start 30 to 50 days after exposure. Many people (especially infants and young children) catch mononucleosis and only have mild symptoms similar to the common cold. Older children and young adults are more likely to have worse symptoms. For this reason, "mono" is more often recognized in high school and college-age students. Symptoms can last from one to several weeks.

Almost everyone catches mononucleosis during their life

As many as 95% of adults in the United States have been infected by the time they are 35 or 40 years old.

No treatment other than rest is needed for most people with mononucleosis

You can prevent EBV infection

Avoid contact with the saliva of someone who has mononucleosis or who recently had it. Keep children's toys clean and out of the mouths of others. Persons with a recent EBV infection should not donate blood.