Infectious mononucleosis is a viral disease that affects certain blood cells.
Most cases occur sporadically. Outbreaks are rare. While most people are exposed to the Epstein-Barr virus sometime in their lives, very few go on to develop the symptoms of infectious mononucleosis. In underdeveloped countries, people are exposed in early childhood where they are likely to develop noticeable symptoms. In developed countries such as the United States, the age of first exposure may be delayed to older childhood and young adult age when symptoms are more likely to result.