Most people who have mono (infectious mononucleosis) will have it only once. But rarely, mononucleosis symptoms may recur months or even years later.

Most cases of mononucleosis are caused by infection with the Epstein-Barr virus (EBV). Once you're infected with EBV, you carry the virus — usually in a dormant state — for the rest of your life. Sometimes, however, the virus may reactivate. When this happens, you're not likely to become ill. Rarely, reactivated EBV may cause illness in people who have weak immune systems, such as those who have AIDS.

Mononucleosis rarely leads to a serious condition called chronic EBV infection. In chronic EBV infection, you have long-lasting symptoms and a viral infection that lasts longer than usual after your original mononucleosis diagnosis.

If you're experiencing signs or symptoms of mononucleosis — such as fatigue, weakness, fever, sore throat and swollen lymph nodes — and you've had mono before, talk to your doctor. He or she can determine the cause of your current symptoms. Keep in mind that many other conditions, such as hepatitis and toxoplasmosis, can imitate the symptoms of mononucleosis.