What Is Mononucleosis? What Causes It?

Mononucleosis is an infectious illness that’s sometimes called mono or “the kissing disease.” While you can get the virus that causes it through kissing, you can also get it in other ways like sharing drinks or utensils. It’s contagious, but you’re less likely to catch mono than other illnesses like the common cold.

Mono isn’t usually a serious illness, but it can lead to complications that in some cases make the disease more dangerous. The symptoms of mono can be mild, but they can also become very severe. If that happens, you may not be able to take part in your normal, daily activities for up to several weeks.


In general, the Epstein-Barr virus (EBV) is what causes mono. It’s a common virus that many people are exposed to as kids. But even if you’re exposed to EBV, it’s not guaranteed that you’ll get mono. It’s possible to be infected with EBV and carry it in your body for your entire life without ever having symptoms of mono.

EBV is part of the herpes virus family, and it’s one of the most common viruses you can get. Most people get infected with it at some point during their lives, and people all over the world get it. In the U.S., about 85% to 90% of adults are infected with the virus by the time they’re age 40. Usually, the infection happens before a child becomes a teen.

EBV is the most common cause of mono, but other viruses can cause it, too.

How It Spreads

EBV spreads through bodily fluids. The most common way it spreads is through saliva, which is why you can get it from kissing. You can also get it if you share food, drinks, or silverware with a person who has it, or if an infected person coughs or sneezes near you. As long as an object -- like a fork or spoon -- that an infected person used is still moist, the virus is probably still present and contagious.

EBV can also spread through blood and semen. So although it’s less likely, you can get mono from medical procedures such as blood transfusions and organ transplants, or through sexual contact.


When Symptoms Start

If you’ve never been infected with EBV and you get it, you may start to have symptoms of mono within about 4 to 7 weeks. You could develop a fever, fatigue, sore throat, swollen lymph nodes, or other symptoms, like sore muscles and a loss of appetite.

Mono can cause different combinations of symptoms in different people. Some people have symptoms that are so mild that they’re almost unnoticeable. Others have no symptoms at all.

Most people who get mono feel better in about 2 to 4 weeks, but sometimes the fatigue can last for several weeks after that. In some cases, it can take 6 months or longer for the symptoms to go away.


If you develop complications from mono, they can be serious. One problem that can happen is an enlarged spleen. If it becomes severe, your spleen can rupture and cause a sudden, sharp pain on the left side of your upper abdomen. If you have pain like this, it’s an emergency. Get medical care right away because you may need surgery.

Mono can also cause complications that affect your liver, including hepatitis or jaundice. It’s less likely, but mono can also cause problems with your heart and nervous system, and anemia.

You’re more likely to develop serious complications from mono if your immune system is compromised because of an illness like HIV/AIDS or because you take certain medications.

Can You Prevent It?

There’s no vaccine to prevent mono. EBV can stay in your saliva for months after you’re infected, so even if you don’t have symptoms or feel sick, you may be able to spread it. This makes it hard to prevent the spread of mono. To lower your chance of getting it, wash your hands often and try not to share such things as drinks or silverware with other people.

WebMD Medical Reference Reviewed by Carol DerSarkissian on October 28, 2018



Mayo Clinic: “Mononucleosis.”

Cleveland Clinic: “Mononucleosis.”

KidsHealth.org: “Mononucleosis.”

CDC: “About Epstein-Barr Virus (EBV).”

© 2018 WebMD, LLC. All rights reserved.