The Epstein-Barr virus (EBV) causes most cases of infectious mononucleosis. When someone gets mono, EBV stays in the body for the rest of that person's life. It can even reappear in the person's saliva (spit) from time to time. But the person probably won't get mono symptoms (like being tired all the time, a sore throat, fever, and loss of appetite) again — at least not from EBV.
If you have mono symptoms — 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.