Infectious mono is a common cause of sore throat among adolescents, where it is called “the kissing disease”.
Symptoms are sore throat, fever, fatigue and sometimes a rash. Since the symptoms are severe and last for weeks, young kids are usually taken to the doctor.
Strep throat is the main problem mono can be confused with. There are blood Tests that can identify mononucleosis that the doctor is likely to perform, and it is important that she does so. The spleen is often enlarged and can rupture. Many other problems can develop.
Mono is most often caused by the Epstein-Barr virus. This nasty double stranded DNA virus has recently been implicated one of the causes of multiple sclerosis, a debilitating neurologic disease.
Please refer to the attached article on mononucleosis by Mayo clinic. Wikipedia will probably give you more information than you want to know, as the links suggest.