Mononucleosis or "mono" is commonly caused by the Epstein-Barr virus (EBV) or, less common, cytomegalovirus (CMV),Toxoplasmosis, Adenoviruses, Hepatitis A, and HIV. Kids infected with EBV are often asymptomatic. Young people and adults who become infected often develop mono. Symptoms like fever, sore throat, and enlarged lymph nodes are present. Mononucleosis causes bad breath until the infections disappear.