Ask the experts
The chances of getting mono a second time are somewhere between next-to-nothing and nothing.
Dr. Hal B. Jenson, an expert on mono and its cause, the Epstein- Barr virus (EBV), has stated that: "Second attacks of infectious mononucleosis caused by EBV have not been documented."
The diagnosis of "mono" is suspected by the doctor based on the appropriate symptoms and signs. "Mono" is confirmed by blood tests while testing to exclude other possible causes of the symptoms. Early in the course of the illness, an increase in a type of white blood cell (lymphocyte) is found. Some of these increased lymphocytes are unusual or "atypical" lymphocytes, which suggest "mono." More specific testing, such as the monospot and heterophile antibody tests, can confirm the diagnosis of "mono." These tests rely on the body's immune system to make measurable antibodies against the EpsteinBarr virus. The current lab testing for mono is a blood test that is not actually measuring the amount of virus in the blood, but is detecting the antibodies in the blood that are directed against this virus. These antibodies can remain in the blood virtually for a lifetime. This does get confusing both for doctors and patients, leading to errant diagnoses of second, third, or forth infections.
Source: HB Jenson, In Nelson Textbook of Pediatrics, 15th edition, edited by RE Behrman, RM Kliegman, and AM Arvin, WB Saunders Company, Phila., 1996.