Mononucleosis recurs in a person when the dormant Epstein-Barr virus that causes the illness reactivates months or years after the initial occurrence, explains Mayo Clinic. Once someone in infected by EBV, it stays in the system for life.
Latent EBV hides in blood cells, and the causes of reactivation normally involve threats to the body’s immune system from other issues. An article in The New York Times states that mononucleosis reactivates if the body's immune system is stressed by other procedures, such as a bone marrow transplant. Symptoms of mono include headaches, loss of appetite, fatigue, and muscle aches or weakness, according to the Cleveland Clinic.