Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Infectious Mononucleosis

What Is It?

Infectious mononucleosis, commonly called mononucleosis, or "mono," is an illness caused by a viral infection, most often the Epstein-Barr virus. Mononucleosis has been nicknamed the "kissing disease" because Epstein-Barr virus commonly is transmitted in saliva during kissing. However, sneezes and coughs also can transmit the virus. In about 15% of cases, mononucleosis is caused by cytomegalovirus and human herpes virus-6, viruses that, like Epstein-Barr virus, are members of the herpes family. HIV infection, hepatitis viruses and even the parasitic infection toxoplasmosis can mimic infectious mononucleosis.

Mononucleosis typically occurs the first time a person is infected with Epstein-Barr virus. Once a person is infected, the virus remains alive in the body for the rest of his or her life. After the initial infection, it rarely produces any kind of illness, but it can be transmitted to others. The initial infection with Epstein-Barr virus does not always cause mononucleosis. It often causes only a mild illness, like a cold, or no illness at all. Epstein-Barr virus permanently infects more than 90% of the people on Earth, but it causes mononucleosis only in a minority of them. In developed nations, such as the United States, mononucleosis most often develops between the ages of 15 and 25, although it can occur at any age.