Mononucleosis. Mono, called the kissing disease. The virus transmitted through saliva, thusly by kissing, you can also be exposed through a cough, asneeze, by sharing a glass or food utensils with someone who has mono in underdeveloped countries, people are exposed to the virus in early childhood, they aren't likely to develop noticeable symptoms. In developed countries exposure is delayed to young adult life.
Developed countries. In many parts of the world, children catch ebv virus when they are still young. At that age, they get mild flu-like symptoms for a few days, and then are well again (they usually do not get a "mono" illness). Because their bodies already carry ebv virus, when they become teens or adults and kiss other people with ebv virus, they do not catch another ebv disease from the other people.