Mononucleosis, mon uh noo klee OH sihs, also called infectious mononucleosis, is a mild infectious disease. It is most commonly caused by the Epstein-Barr (EB) virus, one of the herpesviruses. As with any harmful infection, the body's immune system fights EB virus infection by activating large numbers of lymphocytes, a type of white blood cell. The disease gets its name from these mononuclear (single nucleus) cells.