Mononucleosis

Infectious mononucleosis is a contagious viral disease caused by the Epstein-Barr virus. It is transmitted by direct oral to oral contact and, for this reason, is often called "the kissing disease". In children, the virus often causes no symptoms or mild symptoms similar to the flu. However, in industrialized countries where pre-adolescent oral contact is rare (such as the English speaking countries of Europe and North America) the disease is usually contracted in adolescence or early adulthood, when the symptoms can be temporarily dehabilitating. 
Mononucleosis is generally characterized by a lengthy period of fatigue, although other symptoms such as fever and jaundice are also common. Fatigue can often be lengthy, outlasting the other symptoms which usually disappear within three weeks of first appearing. The disease also remains contagious after symptoms disappear, perhaps up to 18 months, and the virus itself will live indefinitely in the body in a state of latency. 
Mononucleosis will disappear on its own with no treatment. Treatment is for symptoms only, generally ibuprofen to deal with fever and pain. For more severe symptoms, prednisone can be effective.
Back when Hugh Laurie was in college, he was an outstanding rower until he contracted mononucleosis. Instead of returning to rowing, he took up acting as an extra-curricular activity instead.
Infectious mononucleosis at Wikipedia