Infectious mononucleosis (mono) occurs in about 1 in 2,000 people every year. People between the ages of 10 and 24 are most likely to develop symptoms of mono.1, 2
Most people have been exposed to Epstein-Barr virus (EBV), which causes mono, by the time they reach adulthood. They likely won't have symptoms, but they can spread the virus to others every now and then throughout their lifetime.
Citations
Hirsch MS (2007). Herpesvirus infections. In DC Dale, DD Federman, eds., ACP Medicine, section 7, chap. 26. New York: WebMD.
Johannsen EC, et al. (2005). Epstein-Barr virus (infectious mononucleosis). In GL Mandell et al., eds., Mandell, Douglas, and Bennett's Principles and Practice of Infectious Diseases, 6th ed., vol. 2, pp. 1801–1820. Philadelphia: Elsevier.
WebMD Medical Reference from Healthwise