In rare instances, mononucleosis can recur in a person who has experienced the disease before, according to Mayo Clinic. People who have had mono become carriers of the Epstein-Barr virus, which lies dormant and may reactivate later in life, especially in individuals with weakened immune systems.
Mayo Clinic recommends seeing a doctor to anyone who has symptoms of mono and has also had the virus before. The symptoms could be a reemergence of the virus or could indicate other illnesses with similar symptoms, such as hepatitis and toxoplasmosis. Mono that lasts six months or more could indicate a chronic Epstein-Barr virus infection, which is a serious condition.