Causes and Risk Factors of Mononucleosis

Mononucleosis (mono) is usually caused by the Epstein-Barr virus (EBV), although mono-like illnesses are produced by other viruses and organisms. Mono is mainly spread through saliva, which is why it is commonly referred to as the "kissing disease."

Someone with mono may be considered contagious for several months. By adulthood, most people have been infected by EBV but may have had no or only mild symptoms rather than mononucleosis.

mononucleosis risk factors
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Common Causes

Infection by the Epstein-Barr virus (EBV) or, less commonly, cytomegalovirus (CMV), causes mononucleosis. In addition, several other viruses and the parasite Toxoplasma gondii cause illnesses with similar symptoms that may be diagnosed as mononucleosis.

How Mono Spreads: EBV is commonly spread through saliva. Close contact and activities like sharing a cup, straw, or eating utensil can spread EBV. It can also be spread by other bodily fluids including mucus, blood, semen, and vaginal fluids. The spread is usually from someone who is shedding the virus but has no symptoms of it.

Symptoms usually develop four to six weeks after you are exposed to the virus, making it difficult to identify how you acquired the infection.

Prevalence and Age Groups: About half of all children have been infected by EBV before age 5, often with no symptoms or only a mild illness. About 95% of the adult population in the United States has been infected with EBV. Infection most often causes symptoms and illness in teenagers and young adults. If you are a teenager who is infected with the virus without having had it at a younger age, you may develop mononucleosis illness about 25% of the time.

Babies under 1 year old rarely get mono because they receive antibodies from their mother that protect them during several months of life. A mother with an active or reactivated EBV infection can pass the virus to her baby, but this often does not result in symptoms or illness in the baby.

Contagious Period and Recurrence: Researchers aren't entirely sure how long a person with acute mono will remain contagious.

While many will give you the "all clear" sign after six months, a number of studies have shown that there may be a potential for infection for as long as 18 months. This is because the EBV virus may still be active even if you have no symptoms.

Once you have been infected with EBV, you form antibodies. However, it is a type of herpesvirus and, like other viruses in that family, it never leaves your body. After the initial infection has fully resolved, the virus will go into dormancy and will usually remain in a non-infective state.

If your immune response is impaired in the future, however, there is the potential for the virus to reactivate and be contagious to others again. In such cases, you may feel tired or have swollen glands, but be otherwise unaware that you are contagious. At other times, there will be no symptoms. If the virus is actively shedding in the saliva and other bodily fluids, you can transmit EBV to others.

Mononucleosis Doctor Discussion Guide

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

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Lifestyle Risk Factors

Young adults are unlikely to know whether or not they have been infected by EBV as a child. You could be already immune to mono or you could still be at risk for catching it. There is no vaccine and antibody screens are not done.

Precautions to take when you have mono.
Laura Porter / Verywell

It is difficult to prevent mono from spreading, but you can take appropriate care if you or another person has mono (or is recovering from it). It's important to understand that the resolution of mono symptoms does not mean that someone is any less contagious. Because of this, you need to take precautions.

Important Precautions

  • Avoiding kissing
  • Avoiding shared utensils
  • Avoiding shared drinks or drinking straws
  • Covering any coughs or sneezes
  • Washing your hands often

Someone with mono isn't advised to stay home from school or work because of their being infectious. Rather, time off is recommended because of the symptoms they are experiencing.

While oral sex is not considered the predominant mode of mono transmission, research suggests that higher rates of mono are seen in sexually active teens. As such, sexual activity may need to be curbed during the active stages of infection as an added precaution. Protective barriers such as condoms and dental dams can help prevent spreading EBV and are also useful for preventing sexually transmitted infections and pregnancy.

Frequently Asked Questions

  • How long do mono symptoms last?

    In young children, symptoms may last just a few days, while the disease typically lasts two to four weeks in teens and adults. You may be tired for a few weeks, but sometimes the fatigue can last for up to six months.

  • What causes mono to flare up?

    The EBV virus stays in your body in inactive form, so it is possible for it to reactivate. In most cases, you won't notice any symptoms, but it is possible to spread it to others at that time. If you have a weakened immune system, you're more likely to develop mono symptoms when the virus reactivates.

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Centers for Disease Control and Prevention. About Infectious Mononucleosis.

  2. Womack J, Jimenez M. Common questions about infectious mononucleosis. Am Fam Physician. 2015;91(6):372-6.

  3. Crawford DH, Macsween KF, Higgins CD, et al. A cohort study among university students: identification of risk factors for Epstein-Barr virus seroconversion and infectious mononucleosis. Clin Infect Dis. 2006;43(3):276-82. doi:10.1086/505400

  4. Mount Sinai. Mononucleosis.

  5. Cleveland Clinic. Mononucleosis.

Additional Reading

By Kristin Hayes, RN
Kristin Hayes, RN, is a registered nurse specializing in ear, nose, and throat disorders for both adults and children.