Outbreaks are rare.
Adolescents usually report loss of appetite, weakness, and sore muscles. The incubation period for adults is between 10 and 60 days. The incubation period for children and adolescents is between 7 and 4 days. An examination of the blood under a microscope may reveal an increased number of white blood cells known as lymphocytes. Mononucleosis can last from one to several weeks.