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Mononucleosis (mono) is a disease transmitted from one person to another by contact with infected drops of saliva and mucus. Often referred to as “the kissing disease,” it can be spread through kissing, coughing, sneezing or sharing drinking glasses, eating utensils and other items that have had oral contact with an infected person.

In rare cases, mono has also been spread through blood transfusions and bone marrow transplants. Although mono is infectious, it is not as contagious as the common cold and some other infectious diseases.

The Epstein-Barr virus (EBV) causes most cases of mono. cytomegalovirus (CMV) may cause symptoms similar to mononucleosis, although the tests for the condition will be negative. Both viruses are members of the herpes virus family and are sometimes linked to certain other medical conditions. Once a person is infected, the virus remains in the body for a lifetime. However, once exposed to the virus, a person usually develops immunity to EBV. Antibodies produced in response to the virus remember the virus and do not allow subsequent exposure to develop into an infection.

In addition, the virus may occasionally reactivate throughout a person’s life and be intermittently released into their saliva. This causes no symptoms of infection, but may expose others to the virus.  

EBV is most likely to infect people during early childhood. A second infection peak occurs during adolescence. An EBV infection typically does not cause any symptoms in young children. However, adolescents and young adults who become infected with Epstein-Barr often develop symptoms of mono. Approximately 35 to 50 percent of people in this age group develop symptoms of infection, according to the U.S. Centers for Disease Control and Prevention (CDC).

Most adults have either experienced asymptomatic exposure to EBV as young children, or symptomatic exposure as teens or young adults. Thus, most adults have already developed the antibodies that protect them against EBV infection and mono. In the United States, as many as 95 percent of adults between 35 and 40 years of age have already been infected with Epstein-Barr virus, according to the CDC.

Symptomatic mono is more common in countries with generally high levels of hygiene (e.g., the United States), than in the developing world. In regions with poor hygiene, more children are likely to be exposed to the EBV early in life, in which case symptoms usually do not develop or are very mild. On the other hand, in industrialized nations with high levels of hygiene, children may not be exposed to the virus until they are adolescents or young adults, at which point symptoms of mono typically develop.