A two-step infection by the Epstein-Barr virus accounts for the characteristic features of infectious mononucleosis (IM). New serologic tests for viral antigens exist, but a rapid kit test for heterophil antibody usually suffices to confirm the diagnosis. General management is supportive only. Splenic rupture is very rare, almost never fatal if diagnosed early, and, in most cases, is probably best treated by splenectomy. Athletes tend to recover from IM faster than nonathletes. When the spleen returns to normal size, the athlete can return to contact sports, though it may take 3 to 6 months for an elite athlete to regain top form.