The Epstein-Barr virus, frequently referred to as EBV, is a member of the herpesvirus family and is one of the most common human viruses. The virus occurs worldwide, and most people become infected with EBV sometime during their lives. In the United States, as many as 95 percent of adults between 35 and 40 years of age have been infected. Infants become susceptible to EBV as soon as maternal antibody protection (immunity present at birth) disappears.
Many children become infected with EBV, and these infections usually cause no symptoms, or are indistinguishable from the other mild, brief illnesses of childhood. In the United States, and in other developed countries, many people are not infected with EBV during their childhood years. When EBV infection occurs during adolescence or young adulthood, it causes infectious mononucleosis 35 to 50 percent of the time.
The illness is usually spread through saliva and mucus, which is where the "kissing disease" nickname comes from. In addition, mononucleosis can be transmitted in other ways, such as sipping from the same straw or glass as an infected person, or even being close when the person coughs or sneezes.
(Click How Is Mono Spread? for more information.)
When a person becomes infected with the virus that causes mononucleosis, the virus begins to multiply within the body. After four to eight weeks, mono symptoms can begin. The period between becoming infected and the start of symptoms is called the mononucleosis incubation period.