Infectious mononucleosis is a viral illness that affects the blood cells.
Mononucleosis is transmitted by close personal contact involving direct exposure to saliva, as in kissing. Although many people are exposed to the virus that causes mononucleosis at some point in their lives, very few actually develop symptoms. In the United States, the disease is seen more often in high school and college age persons than other age groups. The symptoms include fever, sore throat, and fatigue that may extend from one to several weeks or more. The lymph nodes in the neck may be enlarged and tender.