Mono, also known as mononucleosis, is caused by the Epstein-Barr virus (EBV). Once someone has been infected, the virus stays in the body for the rest of that person's life and may even reappear in the saliva from time to time. However, the person usually won't get mono symptoms (like being tired all the time, a sore throat, fever, and loss of appetite) again — at least not from mono.
If you have symptoms of mono — even if you've had it before — see your doctor. Other illnesses such as strep throat and the flu may cause similar symptoms. Your doctor can make an accurate diagnosis.