Infectious mononucleosis , or " mono " as it is commonly referred to, is a viral illness that causes extreme fatigue , fever , muscle aches, swollen lymph nodes , and sore throat . It is sometimes called “the kissing disease” because it spreads easily through saliva. The Epstein-Barr virus (EBV) causes most cases of mono, although other viruses may cause the illness. Severe cases of mono may cause inflammation of the liver or spleen. If the spleen is enlarged, it is important to avoid vigorous physical activity and contact sports to reduce the risk of spleen rupture.
If the patient has more than 20 percent atypical lymphocytes or more than 50 percent lymphocytes with at least 10 percent atypical lymphocytes, infectious mononucleosis is quite likely, and further confirmation of the diagnosis is not needed. A positive result of a heterophile antibody test also is strong evidence in favor of a diagnosis of infectious mononucleosis. A negative result of an antibody test, particularly during the first week of illness, may indicate that the patient does not have infectious mononucleosis. However, it also could be a false-negative result or could indicate that the patient has an infectious mononucleosis–like syndrome caused by CMV or toxo-plasmosis. The patient should be treated symptomatically, and if the patient does not clinically improve within five to seven days, a second heterophile antibody test should be performed. If an accurate diagnosis is urgently required (for example, in a competitive athlete who wants to return to competition as soon as possible), a VCA-IgM test may be selected. A negative result is strong evidence against the diagnosis of infectious mononucleosis.